former flight attendant here (left the job this past April).

don't believe it's an FAA rule, we were never taught that in training for my airline.

Anyway--i hope some airlines DO charge extra for people who take up nearly two seats. The thing is, it's a hard thing to peg down--it would be very easy to lie about this when purchasing a ticket,you know?

At any rate, if some airlines DO have this policy, complain to the gate agent (not much the flight attendant could do unless there's empty seats).