Typically, Blizzard (and other game developers) will hold off on banning users for cheats, hacks, and exploits until a critical mass of electronic ne're-do-wells are caught. By banning in waves, companies attempt to catch as many people as possible before revealing their hand.
After a previous round of banning, Blizzard explained its position regarding players that were using trainers and other external programs, which some felt didn't warrant bans.
Admittedly, I haven't been playing any StarCraft II online recently so I'm not sure what hacks were being used, but please let us know in the comments if you've seen any foul play recently. Or, you know, if you were banned.