They say this doesn't block ads, it hides them, so publishers still get paid for CPM traffic. I guess they're just compositing the new ads over top of the old ones. In a way, I think that's actually much worse, both for the site owner, and the advertiser. Site owners get lower CTR, and advertisers end up paying for ads that were never really shown.
Agreed. It doesn't matter why you're doing it. Hijacking existing ads is a dirty tactic.