I never understand this defence, its like saying its OK to steal games from a store because they left the door unlocked, and then saying "well another store didn't press charges, so why should tis store?". If I leave my house unlocked, yes its my mistake - but it still isn't OK for people to walk in and take what they want.
If you break the rules, then there is always the chance you will get punished - its really that simple, and its the very reason on all of my accounts that contain items that cost money - I don't break the rules.
Is it shitty the person (or people) got banned, sure it is - but if you take the the chance and break the rules, then you have to accept you may get something shitty in return.