No, there really isn't. It is simply a question of fairness and civility. The simple fact is that if you can't put down the armrest when you sit next to me on a plane, you are intruding on the space I purchased and I am going to be physically uncomfortable for the entire flight. Fortunately this doesn't happen all that often, because even most "obese" people actually fit in the seats, but when it does occur it is truly a miserable experience.