People take advantage of flaws in the way the system is set up. For instance, one common method of cheating is to mark living stones as dead, or to refuse to agree that the game is over by rejecting the results. If the cheating player has more time left on the clock, it is likely that his victim will eventually run out of time and lose.
More sophisticated methods include playing inside the opponent's area, forcing them to continually capture stones until any further moves result in either a reduction of one's own territory or actually filling in one's own eyes and killing oneself.
This could be easily fixed if passing was allowed at all times, but for some reason, if your opponent rejects the result, you actually have to play a stone somewhere - you can't just pass.
The larger question is why anyone would even want to cheat in order to gain meaningless points. In my opinion, the best way to deal with cheaters is just resign immediately and never play them again. Of course, the moderators here frown on that - they think resigning a won game is a worse offense than anything the cheaters are doing, and have been known to suspend people who do so. What's up with that???
