"Fraudulently obtained" could mean that they were keys offered under, say, a cyber-cafe program, or en-masse but with a caveat not to resell, or for developer testing, or just plain stolen, or any number of things.
All of which, sorry, but that's a legitimate reason to revoke if they've then been resold to the public as individual licences.
I have ALWAYS been suspicious of keys from anything other than the original store precisely because of this - you have NO way of knowing if they are genuine or not or whether they could be revoked or not.
And I was always suspicious that a key could be sold cheaper than, say, Steam or Origin or whoever were selling them. That just reeks that it's cheating some rule, even if it's been sold out of region restrictions or whatever - someone, somewhere is losing money and that makes me suspicious why they would allow that to continue.
Sorry, but Ubisoft aren't doing anything "wrong" here, maybe they aren't doing it "right" though (if you know who these keys are, why not put a pop-up on their account next time they login telling them that they don't have a genuine key - but you'll sell them one - maybe at a discount - for X amount of money so that you don't lose on their third-party sales and they get to legitimise their purchase? A bit like MS did with Windows keys a while back?).
Sorry, but the nature of app stores now is that you can't take third-party keys. Hell, I was incredibly suspicious of Humble Bundles but they seem to be genuine and approved and there isn't a lot of money involved on my end if they do get revoked (I'll be annoyed at the inconvenience, but I won't have lost the games as they provide DRM-free downloads too).
When you use an app store, whether that's Steam, or Windows Marketplace or Google Play Store, you have to get the keys from those companies. Ask yourself why would they co-operate with rivals selling their keys at a lower price than in the store, and offer their download and account services to other companies at a loss?
This may be a step backwards in terms of consumer protection, but equally people have been buying cars that only the original manufacturer can service for DECADES. As soon as you fit a non-standard part, the original company just doesn't want to know any more - in terms of warranty, service, etc. I consider this the same problem.
I have over 800 games on Steam. I trust that they are all genuine - lots are via the Steam store itself (most with money earned in-store via selling items from other Steam games!), and the rest are via trusted bundles that do have a relationship with Steam.
Whenever I saw someone fussing about saving a dollar by going through some external game key store that Steam used to obscure the URL to in their forums (always a good sign that they're official!), I wondered when it would catch up with them. I'd much rather have a genuine game for Â£10 than a game that could be revoked at any time without any legal comeback for Â£9. If you don't have the difference to spare, you shouldn't be buying games anyway.
Sorry, but this is your own fault. The only possible excuse is if someone's gifted it to you and you had reason to trust them but I'm not sure if uPlay even has that facility. At least on Steam, all that would happen in that case would be a reversal of the transaction that gave you the game (so you get back anything you traded for it, or nothing if you were stupid enough to trade for it outside of Steam).
Still waiting for all those Russian-sold games to catch up with the people on Steam who bought them cheap, or the games from outside Steam places that people tried to legitimise by using them despite the fact that Steam never say they have a relationship with those companies at all.