Adblock maintains a curated whitelist of approved, unobtrustive ads that most people don't mind seeing (text ads, etc). Last I heard, big companies wanting to submit their ads to the whitelist pay a fee, while small companies can have theirs added to the list for free - presumably this is justified based on the volume of ads that would be added to the list. There's a setting in the Adblock preferences to block even ads on the whitelist.

Some people make it out to be something sinister, potentially because "show unobtrustive ads" is on by default. Given that most people haven't even noticed that they're seeing these ads, I'm not convinced it's a big deal.