The EULA that accompanies delisted Ubisoft games requires players to destroy their purchases. Stop Killing Games is fighting to keep titles working after companies abandon them. A major representative for EU publishers opposes solutions like support for third-party servers.
I can’t find it on GOG’s but I wouldn’t be surprised if it’s in most EULAs. I’ve seen emails saying “confidential, if you are not the intended recipient of this email you must delete it.” There’s no way to enforce that. Ubisoft isn’t coming to your house to review the contents of your drives. I’m guessing it’s to stop some loophole like “you said I can’t resell your game so instead I sold my hard drive (that has the game installed on it)”.