It's not mandatory. It's just become somewhat more difficult to opt out (and you have to be logged in to a Google account to do so).
So it's actually an incredibly clever scheme to get more people to sign up for Google accounts?
I guess, but I don't really see what benefits Google gets from having more people signed up for Google accounts. Perhaps it somehow improves their advertisement targeting?
Bingo. If you just have a Google cookie not associated with your account they can only track your activity on your current computer. If you are logged in they can track your activity on any computer you're logged in to.