cheating, competition, punishment & shame
Microsoft announced it is taking steps to crack down on cheaters, including stripping accounts of their achievements and slapping on an "I'm a dirty cheat" label on the account for all to see. You can read about it on their Major Nelson blog or in the Gamerscore Corrections FAQ.

Cheating goes hand in hand with competition. We like to think that we compete with one another to test ourselves against others, to push ourselves to achieve tangible and intangible rewards, and to experience the excitement of winning when losing is a real possibility. But feeding that competitive impulse also unleashes a less attractive drive to get that top score, to be recognized as the best, sometimes without regard for how that impulse is satisfied.

Cheating is everywhere, from using steroids in the quest to set baseball records to palming an extra $500 bill when acting as the banker in Monopoly to yes, tampering with Gamerscores. We create and use cheats all the time in gaming, with purists preferring to play unaided but many or most of us using cheats both to enhance our enjoyment and to obtain a victory without as much struggle. It's hard to understand how one could derive satisfaction from a bogus victory, but humans are remarkably adept at self-deception as well as the deception of others. Sometimes we regard the willingness to do anything to win as an admirable trait.

"Winning isn't everything, it's the only thing."
"Rules are made to be broken."
"It's only cheating if you get caught."

Good luck with that, Microsoft. We salute your efforts to keep the game clean, but we think it's a losing proposition.