Blizzard Entertainment cannot suffer, it is nefarious tricksters attempting to sneak an advantage in its games through the use of hacks, bots, and other malicious cheats. The company has just confirmed its latest round of mass-bannings in Diablo III, and while no specific number or reasoning is given, the post leans heavily on the rules regarding botting and hacking.
Reiterated in the ban-wave announcement are the things that Blizzard prohibits players from doing with third-party software, including automate gameplay, obtain information about the game that isn't normally available to players, modify D3's game files, and change the way the game interacts with Battle.net.
Blizzard also noted that using hacks and bots isn't a victimless crime; third-party software can sometimes cause big technical issues for both the offender and other players. More info on Diablo III's exploitation policy is available on the official Blizzard support website.