It might be free as in gratis (free beer), but it's not free at all. Your behaviour, your profile, your data: you are the one giving that away in exchange. That data of yours is valuable.
Restricting the legality of advertisements in general might have the nice effect of leveling the playing field, by making it harder for content providers to benefit by-generally covertly-selling out their users (or their users' interests) to such third parties.