What you write is totally different from what article says. That article states that Blizzard also bans people who plays offline/single player games, I'm sure everyone is fine with banning online cheaters.
Longer explanation...
The cheating appear to be people modifying save game files. They alter the file and give themselves more resources, units, etc.
They must have been logged into Battle.net in order for Blizzard to catch them. Single player != Offline in SC2.
The only reason to really cheat is to gain Achievements. As I said before, there are cheat codes built into the game[1], they are perfectly legit, but they do not give you any Achievements. This is why people were cheating - to get the benefit of the legit codes, but still gain Battle.net achievements.
[1] http://sc2.lancraftwc3.com/2010/04/starcraft-2-cheat-codes.h...