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This picture caused a lot of laughter and mocking, when it happened. I've since noticed this sign in many pictures of flight hardware.
It has been my understanding, that satellites are built in cleanrooms anyway. As this memento and others seem to suggest, not everything is assembled in a cleanroom.
So if they don't need the cleanroom, why are there still signs not to touch anything? Most of it should be aluminum, so there is no concern for corrosion, is there?
This answer is speculation, but speculation based in personal experience in satellite clean rooms.
Space hardware is frequently subject to unusually restrictive constraints. Some examples that other answers have also mentioned are that residual oils may outgas and redeposit in vacuum, which can cause problems on optical surfaces, or short electrical panels. Many electrical components are custom built, meaning they aren’t always in protective casings nor as robust as consumer components - this means that electrostatic discharge is a real hazard.
Combine this with the fact that most satellites are built under contract. When the manufacturing company is making their bid, the customer (such as NASA) wants assurance that every component will be carefully certified and protected for damage. Both sides will draft and agree on standard operating procedures to ensure all requirements are met.
These things together mean that uncertified personnel, or even certified personnel not wearing proper protective equipment, are not allowed to touch flight hardware. That is, the very fact that this panel will fly means that visitors cannot be allowed to touch it, even though touching it may not actually be harmful.
I’ve encountered situations where everyone present was fully capable of connecting a certain box, but we still had to stop work and find someone who was certified to do so - these requirements are taken seriously. Failure to follow them brings anything from writeups to loss of future contracts.
Visitors can’t be expected to know requirements like “keep three feet away from flight hardware” without instruction, so the sign was probably placed to 1. confirm that the panel was flight and 2. reinforce the fact that it was off limits. Unfortunately this particular group of visitors appears to have needed better supervision.
Only the person who places a "do not touch" warning knows the true reason the warning was there. "My boss told me to put the warning" might be a popular reason.
As for why hardware in a clean room should not be touched, this also goes on a case-by-case basis. @Uwe mentioned that "surface should be clean for later bondage of tiles on it. But the procedures required cleaning just before bonding anyway", which is plausible explanation.
Sometimes grease on hands may outgass in space and cause degradation of optical characteristics of some surfaces. Sometimes electrical components may be damaged by electrostatic discharges, hence why people should be "grounded" before handling them. Something might just be fragile and break if touched. Something might just need to remain in a given place and not wander around the lab, but the "do not touch" sign sounded more respectable than the "do not displace" one.
As to building in a clean room, if you work with amateur cubesats it may happen that someone touches the solar cells just for fun, but for large satellites I would expect everything to be assembled in a clean room, and be delivered in some protective recipient. Exceptions allowed for things that are cleaned before being worked on.
The object in the picture appears to be a frame of some kind. Usually a "Do not touch" is due to concerns over EM damage, ie, shocking something. The fact that there is no visible electronics indicates that there is probably not a huge risk. Also of some note is the fact that no one is wearing EMI protection gear of any kind, which would make that even more likely to be damaging.
The second major concern is Foreign Object Debris (FOD). FOD simply put is when some piece of dust, trash, etc ends up somewhere it shouldn't be and causes issues. There have been many failures that have occured because something was in a place that it wasn't supposed to be.
In general, most satellites don't require a clean room build them. They do need a controlled environment, EM protection, etc. A cleanroom is only required when biological sensitivity or optics might be a concern.
As far as it can be seen, cleanliness was the issue with the particular piece of equipment, either outgassing or FOD. The equipment in note seems to be some kind of a payload adapter, so my guess is that the primary concern is if a sensitive mission would require the adapter, it would need to be properly cleaned. Just based on the fact that they are not wearing special suits designed for EMI or clean rooms, I rather suspect that the sign was on there for an abundance of caution, likely to get people used to the idea of not touching it without proper training.
