Disabling of redeemed keys happens on Steam.Companies can't revoke the keys because they are already redeemed when the chargeback comes through. The hacker/scammer who uses the stolen credit card gets paid by g2a, g2a takes a cut from the buyer of the key, and the chargeback comes back to the developer who gets nothing and gave away a copy of the key for free.
G2a is a good idea in that you should be able to sell keys or extra keys, but they facilitate a lot of fraud and bad business as well.