The policies on the US carriers have nothing to do with weight restrictions. They just figured that they can fleece the travelling public for more money. And AA instituted this change just about 2 weeks before Christmas last year and not one word was posted to any FF members.

Basically you're allowed up to 50lbs. Between 51-70lbs, you're charged $25, I think. Above 70lbs, your bag would have to go as "cargo". You're allowed a max of 2 checked bags and 1 carryon or 3 checked bags and no carryon. Each additional piece of checked bag costs an extra $50 or $75.