Why is it a recurring theme that Americans have average backhands?
Isner, Harrison, Roddick, Raonic (nearly American) all come to mind as players that have really limited backhands. Exceptions to the rule include Agassi, and possibly, Fish.
Most Americans want to play 'big man tennis' which is dominating with serve and put away forehand.
They view their backhand as merely a damage limitation device - so that they can stay in a rally long enough to use a forehand. Federer, Djokovic and even Murray can do all sorts of things off their own backhands to make their opponent uncomfortable - for them hitting a backhand is not about merely staying in a rally.
Why do you think this is? For a start, the USTA is known to be very quick to mould all players into using a 2hbh, and 1hbh is pretty much banned. But even when you're using 2hbh's, the 'American 2hbh' seems to be especially bad... Mechanically, what is wrong with it?