Developer of Crystal, Dean Murphy, when asked about his decision  to give up some level of control over his ad blocker’s whitelist:

I don’t possess the resources necessary to hand pick, and arbitrate what ads are acceptable. It would require making sure they meet a certain criteria, and I would have to then monitor the ads in order to ensure that standards had been met. It would also require me to form business relationships with advertising networks, etc. It would be a massive time sink and would involve a hell of a lot of work that would distract me from everything else. I’ve spoken to Eyeo, and they have many customers who support their vision. They have set a great standard for acceptable ads, and are backed by some sites that I truly admire.

I’d also suggest reading Dean Murphy’s sort-of FAQ about his partnership with Eyeo and what it means to Crystal users.