Surely it's easier for Google to ask forgiveness than permission. Why wouldn't they do this if they want to track you, especially if they can muster even a shred of plausible deniability to allow them to keep it in place longer?
Have you confirmed that the click through agreements and privacy policies don't mention anything like this?
No …? I'm not sure why I would have; I was just responding to @kerng's surprise at its presence (https://news.ycombinator.com/item?id=23180801).