As for ads - I suppose if there is an ongoing ethics discussion already (and I believe there is), then it's ethical issue. Not sure, though.
It's comparison that's didn't sound right to me. I'm just not sure it's well-comparable, because I suppose the major ethical debate about copyright infringement came from the existence of copyright laws. Which is not the case for ad-blocking.
Although I genuinely wonder what came first - the discussion of ethics of "stealing" books or discussion of ethics of passed copyright laws.