Thanks everyone for your replies.
I don't think it's just an AC rule, but I do fly AC a lot, so I can't say for certain.
In-the-Ear style headphones block out more external sound than over-the-ear style headphones, so that is not an explanation.
Seems to be lots of guesses, but nobody knows the official reason?
Last edited by MrX; Apr 15, 2012 at 9:07 pm