They're not cheating or using any game exploits as far as I know. They're simply making themselves easy kills for their friends and vice versa, and so therefore rank up quicker.
I'd take a booster over a cloak camper any day.
Edit: I read Kozzle's quoted post. Dude, get your wording right. That guy who got 20,000 kills an hour is obviously a cheater, as there's no physically possible way for a player to get that through normal means. A booster on the other hand plays a private match with his friends. He let's his friends kill him easily, then he kills them easily, rince and repeat until an easy rank 50.
I have no major issue with boosters as long as they then don't come onto forums or in game and try to brag or talk smack to others regarding their (inconsequential) stats, which is pathetic enough as is, but even worse when they're entirely fake, and that's what a lot of boosters do it for. To be obnoxious.
The other element is that it stops leaderboards from being a true reflection of anything, which for some people reduces their enjoyment of the game as they can no longer accurately see how they legitimately stack up in the world, and it then reduces the incentive to rank up and the whole XP system, as if it's no longer a reflection of your ability and can easily be circumvented, then what's the point? Where's the reward anymore? Would you go to work if cash machines suddenly started printing free money?
I don't play for those reasons, but a lot do, and by and large, boosters tend to be people with bad attitudes anyway, so I have no sympathy for them getting deranked for circumventing how the game was meant to be played. Banning is too harsh as they're not hacking, but deranking is appropriate.