Ubisoft aren't as dumb as you think. They know that when they ban these keys most of the people who bought them will blame the vendor for selling them a dodgy copy.
I'm not sure they will do that. I think the majority of the gaming press will flame them for doing this (and rightly so, you don't punish people who are trying to be your customers, even if they are seeking bargains) and I think the majority of customers will feel however they are told to feel. And I think most of the rest of them will be pissed off because they won't have been able to play the game they paid for.
There's often legitimate discounts on games, so there's no valid reason to penalize customers for seeking discount prices. Likely some of those users made their purchases in ill faith, but I'd bet they were in the minority.