I'm presuming that they aren't banning people for the built-in cheats? (Which disable Achievements if you use them apparently...)
And I bet if anyone tried to take this to court, Blizzard would bang on about how "memory address injection/modification is a violation of the DMCA" or some such bollocks.
Correct, the built in singleplayer cheats just disable achievements.
The sad thing is that they have already fought for such things; and won. :|
I think the main problem is Blizz are really trying really hard to push Battle.net.
And as achievements and score etc tie into this so much..
Yeah, they claim that they are targeting people who are using trainers to get the achievements easily, The achievements provide you with a score that everyone on battle.net can see. They also unlock character portraits for your avatar.
That being said, its no excuse from completely banning someone from playing the game at all. They could just disable the persons achievements permanently (even that is quite harsh in my opinion.)