Apparently, Valve had quietly implemented an account policy to allow banned players to access their games months ago.
Valve’s apparently updated its account policies to allow banned players to access their games. Previously, users who had been banned from their Steam account were locked out from playing their games, bringing up complaints about Steam’s DRM.
Valve’s change in policy was quietly implemented, according to a recent forum post by Steam forums Volunteer Moderator Heffebaycay, about two months ago. Heffebaycay’s response spawned out of a thread regarding EA’s recent policy change also allowing banned players to access their game.
Under the new policy, banned Steam members no longer have their accounts disabled but now have “account locking” set, meaning that while they’re able to access their games, they will be unable to buy games, redeem codes, or trade items.

I don't think this is a good idea...
EA only said yes to letting them back because they NEED people to use Origin. If it doesn't get enough *ahem* steam, then it'll never pickup and it'd fail as the online system they want it to be. There really is no reason why Steam should do the same. I can only imagine they allowed EA to implement their own terms for their specific games on Steam as part of an agreement into allowing Steam to sell their games. If not, then Steam is being insanely silly. If anything they should be increasing the effectiveness of their anti-cheat system that's already in place (VAC), and making it so it's more feasible for ALL the games they host that involve online play.