In principle this may not be a very important distinction. However, I often think that I would stop blocking ads _IF_ the website I was looking at had actually vetted the ad I was seeing. I do not want to see so much garbage just for visiting a website. For example: I block all ads from theverge.com, not because their content is bad, but because I find the outbrain ads at the bottom of the page so asinine and tasteless that I'd rather not see any ads.
To continue on with theverge example, If they actually had selected the individual ads they were placing, I'd be totally fine with that. But I am not fine with them delegating that responsibility to another company that clearly is not up to handling the task.