How do you know that. Ubisoft has given no information as to exactly why the keys were revoked. It could be that they were bought with stolen credit card numbers or it could be that the keys were just sold from non-official resellers but are still perfectly valid keys.
Even if the keys are sold by random people through various markets like G2A's marketplace that still doesn't mean the keys were stolen. As others have said they could be keys that weren't being used by people that bought new video cards that game with free games, or they could be keys bought in regions with cheaper prices. None of that is illegal though it is certainly frowned upon. The region issue can cause problems if a game is region locked but clearly if the people were playing for up to a year that couldn't be the case.
Until Ubisoft comes out with an official statement we can't know if the keys were 'stolen' or just bought from an un-official marketplace.