Much as I would love to portray Blizzard as Goliath to the cheat-makers' David, what they are doing is not only legal, but kind of right. As far as I can tell, they are going after the guys who made money by selling these hacks and trainers. If they were giving them away, I'd say this is overboard, but they were profiting by selling unlicensed merchandise that is derivative of copyrighted work.
Second, saying that some people were only using the cheats in single player seems pretty thin, considering the game already has plenty of cheats built into it, which automatically disable the achievements. I know, many players don't care about achievements, but it's still part of the game.
Now, if Blizzard were suing the players who they've banned, I would say they're acting evil-empire-ish. But they're not suing fans, they're suing people who unlawfully tampered with their intellectual property and made money doing it. Did this really cause lost revenue for the company? Possibly, but that will be for the courts to decide.