Essentially they are writing and maintaining a anti-anti-circumvention tool. I know it might sound crazy, but think about this example-
Bob writes a computer game. He wants it to be free for registered users, but guests can either pay a monthly subscription or be ad supported to use it as well.
Joe writes a crack for said game, so everyone gets the paid experience.
I think anyone would agree the above scenario is a prime case for a DMCA lawsuit as it is the definition of a circumvention of access control.
I do not see how it would be different for doing the same thing on a website. In the eyes of the law, they are both software getting circumvented. Adblock plus is developing and maintaining ways to circumvent access controls developed by other companies.
Personally I dont have a issue with adblock and the generic filters. I have a issue with the site specific filters as I think they are likely illegal depending on each sites tos