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.