I see your point. However I think you will find that most AC admins will be able to see it wasn't you. PBBans records the IP address of everyone who uses a GUID on their servers, and we would be able to see if it was triggered by someone else. In most "stolen key" situations it is very obvious because you will see IPs of completely different countries than the original user, and the violation that triggered the ban is probably not that of the real user. In which case he would need to buy a new CD key, but his reputation would be intact, even if it had little spot on it.
In the case of a serial number being leaked and being proactively banned, I think you will find this is something that EvenBalance presents as a selling point to the game publisher. Gaming companies don't care as much about cheating as they do piracy, so you have to give them some sort of incentive to go with their Punkbuster product. As far as we care it at least proactively prevents some cheating because quite frankly, if a key is public it is going to be cheated with before long.