You're all looking at this the wrong way. Number one, running an item scam on TF2 could very well mean someone is conning people out of money; a Ponzi scheme if you will. With that being possible, it makes sense for them to ban until the issue is sorted out. And as stated before, this is just one case. Not to mention a case where the user in question admitted to being a greedy jerk.
To those worried about abuse: Valve has never abused their powers, because if they ever did, it would be all over the net, and kill their profits quick.
To those worried about griefers: I've been playing TF2 since '07, and never had any such problems, even with the new trading system, nor have I heard of anyone else having these sorts of issues.
It's really silly to see how many people are getting riled up over one incident where the man who was banned admitted he was playing foul: did you guys even read the link?