Some of the best game composers are Americans.
Trent Reznor just knocked the quake 1 soundtrack out of the city, it's the very best of all ambient soundtracks, by my opinion.
Jeremy Soule has made a lot of very mediocre efforts, but he's also got some standouts, like Secret of Evermore. He started young and writes tons of stuff.
Laura Barratt is a virtual unknown, she made a lot of incredible songs for Simtex including the Moo2 soundtrack. (Did you know Steve Barcia worked on both Master of Orion/Magic and Metroid Prime? This blew my mind since they're both such powerful sci-fi experiences)
And so forth, there are simply too many to list. And countless Japanese as well, Uematsu, Koji Kondo, Kenji Yamamoto, etc. The woman who wrote a lot of significant Castlevania 1 sountracks, Kinuyo Yamashita, (one of the most powerful of all game soundtracks) continued to compose, but nothing else she did ever stood out that much. MMX3 was impressive, and as good as the original, but she followed the existing style very closely.
I wish I had the ability to understand music. It affects me, possibly in a way that is unlike what the composer was experiencing himself. But to create it, I simply feel nothing, I hear nothing. My internal sound is not rhythmic or harmonic.