Surely you can target adverts based off of searched terms (as in immediate, not past), current site(eg: the article and not the reader).

Beyond that you get into grey territory, and most of the time down right predatory.

I can run a campaign based on location of search and keywords, that's fine for me. I don't need the user user data. That information is non specific in relation to an actual user. What excuse does anyone have for needing a users history?