Those people were probably banned a long time ago. The leaderboards take time to clear out. Besides, everyone thinks anyone above level 40 is a cheater, so it doesn't really matter if you're on top or not, as you will be known as a cheater, legit or not.
Bungie has done all they can to stop cheaters. You can only enact so many countermeasures, and even then, hackers are a smart bunch(for the most part), and will eventually find a way to get past anti-hacking patches. They can't go to everyone's house and inspect their Xbox for mods. I'm sure there will be more patches and improvements to the anti-cheating features we have now, but they take time to develop, test, and implement, so you're going to have to be patient. Besides, wouldn't you rather have Bungie spend their time on their next game?
And yes, this belongs in Zanzibar, but these threads have been made countless times, so try not to make any more.
[Edited on 9/20/2005]