I suspect this "enforcers" thing isn't going to be too much of a problem. Mars is tough to live on and hard to get to, and it will remain that way for a few centuries at least.
Living as we do in a world where the CIA can track down and kill you if they so much as lift as a finger, it's hard for us to remember just how tenuous a grasp any Earth government would have on a Martian colony. Remember, until the 19th century, when communications speeded up, about all European countries cared about was that their colonies bring home the goods.
Simply put: the remoteness of Mars may also be its salvation. While American law may be the foundation for the colonists' legal system, it's likely that they'll have- if only de facto- a large amount of freedom in which of those laws they choose and don't choose to follow.
To boot, there are a number of very well-grounded concepts in American law that simply won't work. If anyone thinks that Mars is where we'll send the world's John Galts to create a Randian paradise, they should chuck their ticket in the rubbish bin. Mars will have a legal and social system that is well-geared to people in a hostile wilderness trying to make a living: if there is capitalism, it will not be corporate, and it will be heavily regulated; the government will likely be democratic, but it will be highly centralized and possibly strike even moderate to moderate liberal Americans as overly controlling. As well, it's likely that the first Martians will find that democracy as she is practiced in the modern West is too change-happy to be of much use. America can put up with a good amount of gridlock, and has been, but on Mars the sort of gridlock we've been seeing would be suicide. How they'll fix those problems, I don't know, but they're certainly going to have to strike out on their own and not operate according to the American way of doing things. (This is where I think having the Chinese on board for this project would be a good thing. I'd much rather have the smartest guys on the ship running the show than whichever fool demagogue gets up on the stump. But I think that individual civil freedoms will still be really important.)
What will the American/Chinese response to all this be? Likely, nothing. It would look really very stupid to- after spending who knows how much on a mars colony- spending as much all over again to conquer it for not sending music companies their royalties and making political candidates pass a test, or whatever they do. But nothing advances technology like a good war, and if we spend a hundred billion dollars finding out how to send eighty thousand people to Mars peacefully, you can bet we'll spend five hundred billion dollars finding out how to send as many troops even faster to conquer them.
But even there, there might be trouble. It's much, much easier to get to Earth from Mars than vice versa, because of the gravity well, and we all know defenders have a built-in advantage...