When you compare the waste products of LOX/Kerosene vs hydrazine/MMDH/etc, it is actually not too terrible at full combustion. (End up with CO2 and water, or CO2, Ammonia, and water).
However, if say, a fully fueled Proton looses control say 50 seconds into flight and collides with the ground, literally a million lbs of hydrazine is the definition on Unenvironmentally friendly. While the Proton is very reliable, this has happened twice recently, once in July 2013 and again in May 2014.
Also since Russia has no good coastal launch sites, they have no choice but to drop stages that never have completely used all their fuel over land, which is no fun wherever it lands. (Mostly in barren desert).
It is fundamentally the difference in the fuels.
Now technically LOX/H2 is better than LOX/Kerosene, but Hydrazine is so bad, it is worthy of the designation for switching to Kerosene.