Just because a player is in position to receive a kick doesn't mean he has to stand at attention with his arms at his sides. He just needs to be careful that whatever he's doing - shading his eyes, signalling poison, scratching himself - does not resemble a fair catch signal.
This is one of those situations where the rule book isn't doing us any favors. According to the book, keeping the receiver's arms at his sides is actually a pretty good idea because:
“ART. 4 . . . An invalid fair-catch signal is
any (emphasis added)
signal by a receiver before the kick is caught or recovered:
a. That does not meet the requirements of a valid signal.”
So if a coach who knows this rule asks about the "poison signal" we have to say "Well coach, I know the rule says ANY signal, but that's not what it really means."
Or, we throw the flag and look bad because "everybody knows" the receiver meant Stay Away.