Airline says all backpacks must be under seat, but its my only piece of luggage

Hey, everyone wants as much legroom as possible. Even if I don't have a standard "suitcase shaped" bag with me, I have been told ON OCCASION that my backpack has to go under the seat ahead. I feel I've been put in a sub-standard class, hay, I've only got one bag, I'm carrying less than all these other people with 2 coats and duty free bags and other stuff. Why cant they just leave me alone?