No... they verify you own the game, because you have a serial key and that check is installed in the base level version of the game on CD (however you bought it, that initial check for valid serials is included internally. DLC gets added later, and verified externally)
Some are added entirely with the same key (complete editions and stuff like that). This is a problem of Ubisoft simply being bad, Steam for example uses the same key system for games or dlc or whatever you buy on the platform (or outside stores)
Again, you do not own the game or the DLC. You simply license the rights to access the publisher's property. Licenses can be revoked.
Well, guess I'll shamelessly pirate whatever they ever decide to revoke.