I actually just replied to this (more or less) in a very similar thread;
This entire step may feel a bit excessive to some, but I wish steps like these were taken by other companies within their games, more particularly within the online portion.Though I'm surprised people have been banned for cheating in single-player, especially without any sort of warning, I can understand, to at least some extent.
Although some people don't care about achievements or unlockables, a lot of people do. And it certainly ruins the experience for other users when someone has unlocked or achieved everything in an extremely short period of time, or entirely through cheats. The main reason users dedicate extra time and energy into a game is to achieve or unlock things. They can then feel a sense of accomplishment once they're rewarded.
I feel it's a lot similar to MW2. You can't play the game for more than ten minutes without getting a message saying "Hosting a tenth prestige lobby for 800 MS points" (Yes, I'm referring to the 360 version) which in itself is entirely annoying, but also quite discouraging if you're someone who actually wants to reach the tenth prestige. It sucks to know people are paying for, or cheating to get the 'ultimate' or final achievement(s) in a game. And the more cheaters that go unpunished, the less some people will want to play, especially when it comes to online gameplay.