The problem is that the US government gets really bitchy about people shooting at even fake Americans. Remember the whole Medal of Honor deal where people convinced themselves that it was a Taliban training simulator? Yeah, our government is pretty stupid sometimes.
Reasons why it probably wouldn't work aside, I think that it's a fine idea. I feel that a game based on the Vietnam War could be really intriguing if both sides of the conflict were represented--it could provide some interesting gameplay variety while providing a more objective view on a conflict that fell into a bunch of moral gray areas. That's really where I fell like a game with Americans as the bad guys would shine--the conflicts which didn't have a definitive right or wrong side (it's hard to argue that the Nazis were just misunderstood and pitiable, but it's equally as hard to claim that Vietnamese farmers were a force of evil).
I suppose that what I'm trying to say is that someone should come up with an almost documentary-style game which fully fleshes out both sides of a conflict and serves to educate. I particularly think that the Vietnam War would be a great example of this, being a rather fascinating war in terms of both politics and tactics, but just about any conflict (even a made up one) could provide an interesting point/counterpoint system by letting players fight both as and against American troops.
Random other thought: I'm beginning to wonder why Americans are the only people who seem to get to star in games these days. Where are our games starring the British, German, Italian, Russian, Chinese, Japanese, etc.? Why is it that pretty much every game stars a generic US special forces commando? I mean, we can't be the only country with interesting conflicts in our past and interesting political climates now, so why is it that games keep falling into the 'US vs. Nazis' or "US vs. Russians and residents of an unspecified Middle Eastern country'? Let's see more games about other countries, the USA is boring.