Google tried to get users to link their gmail accounts and youtube accounts. Thus, if someone sent you a youtube link in gmail and you clicked it - you would be logged into youtube already/automatically so that they can track your viewing behavior.
Actually, that was the whole point. They were not allowed to due to their separate service agreements/privacy policies.
Then, early in Q1 2012, presumably as part of the whole G+ ecosystem push, they unified/changed their Privacy Policy terms across all (most?) services which users also had to accept.
