I don't think it's an American thing...for once. I think it just so happens we had a few guys with weaker backhands and maybe not great technique. But Fish, DY, Querrey all have backhands that seem very good to me(as JRoger posted above).
And Sampras' backhand was a bit underrated.
I think it's just what they are naturally comfortable with too. Kei was trianed in the US and his two-hander is stellar.
Racquet with RPM Blast.