Stephen Toulouse, in charge of Xbox Live Policy and "Enforcement", has explained Microsoft's stance on dealing with those dirty, Gamerscore cheats.
Speaking on Major Nelson's podcast, he explained that "through a combination of profile resigning - which is against our terms of use - or what's known as glitching," people would "try to trade profiles as fast as you can to confuse the console and it might pass an achievement."
But if you earn an achievement while you're not actually playing the game, that's against MS's terms of use.
When asked if Gamerscores could be reset over just one game, Toulouse said, "I'm going to go ahead and admit that the bar is a little bit higher than that, because we are a little bit conservative. It resets your score to zero but it also puts a tag that you've been caught cheating on your Gamercard. Since that's a pretty big scarlet letter, we want to make sure that we're absolutely sure."