I was watching a nature show and the reporter was doing the interview from a helicopter. The window he was setting by had a big red sign above it that read:
DO NOT TOUCH WINDOW
Why not, what would happen if you did?
My guess - you’d smudge it. And the pilot wants as much of an unimpaired view out the windows as he can get.
A helicopter’s rotor is like a very large, powerful whole-house fan. If you touch the window, the helicopter will implode.
[mod note]
This is the GQ forum, where we post factual answers to questions. Please save the snarky answers until the question has been answered.
[/mod note]
I should have added this was a side window behind the pilot. It’s possible I suppose but it seems unlikely the pilot could use this window for navigation.
Maybe they just don’t want you to lean against the window, but a “do not touch” sign is less ambiguous and safer.
This might be the reason (from a Bell model 212):
*Windows - do not touch, they are designed to be pushed out in an emergency. *
Then the helicopter implodes! 
Only after Schwartzenegger leaps from an airborne motorcycle onto the helicopter and attacks the pilot.
This was going to be my answer.
Of course the helicopters I’ve flown aren’t so fancy. Two-seaters, and the side windows are in the doors – which were usually off anyway.