Piracy

I mean, cdkeys.com doesn't allow third-party sellers and (supposedly) sources all their keys from verifiably legal sources, usually just region arbitrage. Considering they come into ownership of all the keys they sell, I'd think they lack all the "safe harbor" protections of the others.

Thing is, cdkeys.com is about the same price as the others. Which suggests to me that the "stolen keys" rate from those others is lower than some companies would have you believe. Remember, legal or not, the big label stance on all this is an extension of their stance on buying used, which is that they would rather you pirate something than support even a legitimate third-party or cross-regional market.