Blizzard isn't banning players who are just cheating in single player, they are banning people who use trainers to modify the game client. Blizzard allows you to use official cheat codes that, when activated, disable achievements. The trainers which modify the game client do not disable achievements. I may not care about achievements, you may not care about achievements, but there are probably some people out there who do care about how many achievements they have compared to everyone else. It's part of the game experience that Blizzard is offering, and they are trying to protect its integrity.
In any event, modifying the game client is against the EULA you agree to you play the game for the first time. it should also be pointed out that these bans are not permanent bans, but 14-day bans.