Those things within our control we can. We can chose to do this responsibly or not.
So we should not go shopping, or have celebration dinners with the locals or talk to the locals unless strictly mission related. Every interaction affects the worlds we visit and if you think we shouldn't leave a footprint, then we should stay isolated as much as possible, but no one has done that, not even you. So why is all of those things that could have been easily avoided and were irrelevant to the mission okay, but magic that's closely related to the mission is your line in the sand?
Also tell me in specific terms how this would work? Who could make people see something different on this side of the camp? Who would continue the illusion on the other side of the portal? What happens if the barrack is attack and people lose focus and the portal is seen? These are not rhetorical questions. You're suggesting a plan of action, how do you think we should implement it?
Again it seems that responsible means 'don't tell them about magic', but not 'hey I want to go to the pub' or 'let's se what books they have in this town'.