They're not stolen CODES, they're simply purchased with stolen cards. And if you think it's not a big business, you're a fool. It's far easier to obtain and sell a digital product before the card owner realizes it than a physical product. They may purchase the keys from a lower-price territory, but that's not necessary since the original purchase price doesn't matter to the buyer, they're making 100% profit no matter what.
The "working man trying to catch a break" line is a terrible excuse. Altho, unlike people selling tvs from the back of a van, you're purchasing from an e-tailer who can make heir site look as pretty as any legit key seller. The site may normally BE a legit key seller, but they bought up a bunch of cheap keys and didn't ask where they came from. And with Steam offering crazy deals on games (altho usually not brand new ones), it could be difficult to tell if the site is legit.
I'm not sure revoking the keys is a good idea, but it DOES lower the chance that you'll buy from that vendor again, which is their end goal. With a game packed with micro transactions, I'm not sure how the dev loses on this one. When the new Mortal Kombat had keys available for $15 the week it came out, THAT should have raised some red flags. Oddly, didn't hear anything about bans for that one.