To the OP..........
Well, since I am an American, and I used to race motocross and currently road race motorcycles ( and am quite good at it at the local/regional level anyway ) as well as the fact that I drag race one of my bikes and do alot of street riding............. - you saying Americans can't ride " motorbikes" is a bit of an understatement.
That said, although I have not yet watched Moto GP from Silverstone, I can imagine that the Americans didn't do so well, seeing that as far as I know , Nicky Hayden is the ONLY American in the class/on the grid and he does not fair too well.
But comparing Moto GP which is the elite of the elite of the world, to American riders in general is not a fair comparison.
What kind of "motorbike" riding/racing do you do ?