The argument is that they're not blocking all ads, but they're blocking ads of poor quality, are overtly disruptive (ostensibly the name of the game in advertising) or are potentially harmful to users.
Which is why you can opt in/out of things like Google Adwords.
Adblock Plus has created a group of 'guide lines', companies who agree to follow their guide lines will have their ad's hosted.
The problem is every company does not want to work directly the Adblock Plus. So a company (Pagefair) created adblock plus get around as a service, where if you work with them they get your ads around adblock by registering with Pagefair and adblock plus signing off on 99.5% of all all pagefair ads.
Basically adblock is attempting to create a standard for ads, or at least sells that they are. The problem is the volume of ads that PageFair hosts are beyond the review of single entity. There is no intensive to review these ads, as it would hurt their income AND require more labor (or at least require an analytic investment).
Ultimately all that Adblock Plus does is give Eyeo a monopoly on ads.
Presumably if Pagefair, in their role as curator, allows too many shitty ads under their wing then Adblock Plus will block Pagefair, and pretty much destroy their business. They do have an incentive.
Which is why you can opt in/out of things like Google Adwords.