First, no that's not the reason it is one way, though it is something that is a matter of concern that has to be thought about.
It's for reasons of cost and technology. Mars is a much harder place to land on and take off from than the Moon. That's because of the rocket equation
The delta v to escape from Mars to orbit is 4.1 km / sec. To escape from the Moon is 1.6 km / sec.
That's only 2.5625 times faster. But because of the rocket equation it makes a huge difference. You have to multiply the mass you need, to get into orbit by e^(deltaV/ExhaustV) to get the mass ratio. So if you multiply deltaV by 2.5625, keeping everything else the same, that means the mass ratio is not just multiplied by 2.5625, it's actually taken to the power of 2.5625.
Example, lunar module ascent stage had dry mass of 2.15 tons and including fuel, 4.7 tons. So the ratio was 2.186. For Mars you have to take that to the power 2.5625 so that would be (4.7/2.15)^2.5625 or 7.412 approx. That would make the total including fuel a little short of 16 tons. That's a rough first idea of course. Nothing like as difficult as a Saturn V but much much harder than a lunar module.
Critics of Mars One say their budget to land on Mars is already unrealistically low given the number of new technologies that would need to be developed for the mission. Landing on Mars is far harder than landing anywhere in the solar system - because of the high gravity again. The thin atmosphere lets you slow down from Mach 4 down to Mach 1 but is not enough to get you to a comfortable soft landing, as even with parachutes, at least the largest tested so far, you hit the ground at over 200 mph. That's why you have the need for technologies like the sky crane. It also makes landings far less accurate than they are on the Moon.
So their idea is to make it easier by leaving out the ability to return, but it's not clear that that does reduce the level of technology that significantly as it's landing on the surface safely that's by far the hardest part of a journey to Mars. But it is one less thing to develop.
However - there is indeed significant risk of contamination both ways. Several studies and papers back this up.
Some back contamination issues are:
Pathogens can evolve independently of any higher animal host e.g. as a disease of microbes (as is true for instance for Legionaire's disease - a disease of amoeba that jumped straight to humans quite recently).
DNA of archaea can be shared via gene transfer agents - if those got into the sea, then one study found that almost half the
micro-organisms in a sample of ordinary sea water took up the genes in a GTA that conferred antibiotic resistance by the next day, even with no selection advantage to take it up.
A new micro-organism could disrupt natural cycles, displace other microbes in an ecosystem, be a disease of our food, or a disease of other organisms on land or in the sea. So it doesn't need to be a disease of humans to be a problem.
In the most interesting case, independently evolved, it may not be based on DNA. In that case, then the XNA (so called) could be hazardous if it out competes DNA or can occupy niches not occupied by DNA.
Also if it is XNA, then our immune systems and those of other creatures may not recognize the chemicals it produces as the signature of life. This was a concern raised by the Nobel prize winning microbiologist Joshua Lederberg.
In that last case, it could just colonize parts of our bodies, our skin, intestines, or whatever and our bodies only respond to any physical trauma it causes and are not adapted to try to do anything about their cause.
It could harm us directly, or it could produce chemicals that are harmful to us as byproducts. It's in the interest of microbes that colonize our bodies to keep us alive, as their host, indeed many eventually adapt to be symbiotes, e.g. the microbes we use to digest food, essential to our survival. So it's not in a microbe's interest to kill us, but could easily do so just because it is not adapted to us, or us to them.
So - life from Mars - could co-exist peacefully, be so harmless we could eat kgs with no harm, could be unable to survive on Earth - but it could also be hazardous to Earth life.
There has been meteorite exchange between Mars and Earth but most of that happened billions of years ago and most microbes probably can't be transmitted between planets on meteorites.
It is believed that some could (though not known for sure if any did) - but there is plenty of room for the possibility that life on Mars is significantly different from Earth life.
The other way - if we introduce Earth life to Mars then we risk contaminating a pristine planet irreversibly. This closes off any future options that might involve a Mars with no Earth life or with different lifeforms there. I would argue strongly that we shouldn't do that until we know very well exactly what Mars is like and have a good idea of the effect of our actions.
It might make any Martian present day life extinct and confuse the study of past life on Mars. Also, it could cause problems for future terraforming or whatever we might decide to do with Mars, if anything.
You can read a lot more about it in my articles on the subject:
Case For Moon - New Positive Future For Humans In Space - Open Ended With Planetary Protection At Its Heart
Need For Caution For An Early Mars Sample Return - Opinion Piece
Trouble with terraforming Mars
and other articles in my column.
In forward direction, introducing any life to Mars would hugely confuse attempts at scientific investigation.
See Could Microbes Transferred On Spacecraft Harm Mars Or Earth - Zubrin's Argument Revisted
On searching for life by looking for changes in the environment caused by life - that's okay if life is abundant. But it's no good if it is rare.
Life on Mars is likely to be rarer than life in the Atacama deserts or McMurdo dry valleys - and there - life is so hard to spot that in some places life was only detected for the first time in the last decade.
You are talking about microbes that may have lifetimes of a thousand years or more, slowly metabolizing, hardly making any changes to their environment, just a few of them eeking out a living very much on the edge of what's possible.
We have instruments that can detect life even in such challenging environments as those, and they could work on Mars also - able to detect a single amino acid in a gram of soil for instance, and find its chirality. For that matter the Viking labelled release - updated to take account of the confusing chemistry on Mars - is also exquisitely sensitive able to detect metabolic activity of a few non reproducing microbes.
But could be a long search as there are many places on Mars where we can search, at least four different main types of habitat - the deliquescing salts (within top two cms of soil), the warm seasonal flows (whatever they are, nobody knows for sure but seems pretty convincing is water in some form), the solid state greenhouse effect melting layers of ice - it is same thing that causes the Martian geysers by evaporating dry ice - same thing could also melt ice and may be a habitat and may explain the dark dune spots - and then also life may be able to live using photosynthesis and just the humidity of the night time air even in equatorial regions.
May be life, very low concentration, in many places. Or may be very rare just here and there. And you may have habitats that life could survive in which it just hasn't reached yet, because on Mars it would surely take longer for life to spread to new habitats, and instead of colonizing within weeks or months of habitat formation, may not happen for thousands or even millions of years depending how the life spreads and reproduces and if it can e.g. be spread in the Mars dust storms, say, or needs some other rarer or slower method of transfer.
On reverse contamination
More on reverse contamination
On the general approach of Moon first and taking an open ended approach with planetary protection and reversible biological exploration as core principles.
(edited some more, Robert)