My thinking is along the line that it costs someone to create, maintain and run a website, and if that site is free to users, who pays these costs?
It is like TV -- in Europe one buys a TV and a license to receive program content but there are no ads because the licenses cover the costs (or at least that is the way it was when I lived there). In the US we pay for the TV, but the content is free to users, paid for by the advertising. The Net is no different, IMO -- it is mostly free, and the ads pay for the costs to create and maintain sites free to users. Personally I prefer this way to having to pay for a license to use the internet, or pay for each site one visits.
Newly