Primary example: the Like button. Even when I'm not logged into Facebook the button tracks where I go. I never checked a box to have the like button shown to me.

Also, agreeing to use a website does NOT give the website the ability to break the law. Otherwise, we could have drug-trafficking sites completely in the open with a box saying "By checking this box, you understand that we sell extremely illegal drugs, and that you will not take any action against this site."