People who block ads are a minority. Sites that serve heavy content like video would care if someone wastes their resources but blocks ads, but why would a site that serves a few KBs of text spend the resources on blocking such users or making the ads beat the ad blocker in a tiresome cat and mouse game?

Those users could even share or recommend the site to someone else who doesn't use ad blockers, so it actually makes sense to not try to battle ad blockers if you want to make your site more popular.

This makes sense for sites that rely on network effects, like forums or classified ad sites and so on. Unless they have a near monopoly or some really valuable content, they would benefit financially if they let people block their ads.

I can't back that up with data or anything, but it makes sense to me.