Most free web sites pay for their upkeep with ads. It has been an unwritten agreement since forever (or at least as long as there have been ads on the web) that if you consume the content, you pay the creator by looking at the ads on their site.
Consuming the content without looking at the ads is like shoplifting because you don’t like the way a store’s checkout counter works and/or the fact that they want money from you at all.
I’ve been on the web since my college installed Mosaic on their HP-UX machines. I wanna say summer of '94. Thus, I can honestly say that I’ve seen it before the first commercial banner ad was sold later that year. I actually thought ad were worse in the early 2000’s than they are now. Flash should never have been used for that, for example. My main problem with ads these days is that there are sites where the signal/noise ratio is just ridiculously bad. In those cases, I vote with my feet and stay away.