And IMHO even more importantly, with advertising the dynamic between content provider and user completely changes. The user no longer primarily is a customer, but rather becomes the product for the advertisers. The content provider no longer only has to cater to the user, but also to the advertiser, with the conflicts of interest that can bring (e.g. unpopularity of critical product reviews).

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.