If you've bought into the lexicon of marketing it is. They're stealing revenue from you.

If you haven't, you're an asshole and you're fucking your users for no reason at all (ads are no reason).

I agree the position I suggested is only a plausible assumption rather than a self-evident truth. Even so, I think it's a reasonable premise. Most people aren't going to use an ad blocker and then immediately leave your site just because of some related message, so I don't see the search engine issue as much of a danger. You might lose a few people who might have clicked on your ads if someone hostile spreads the word around, but firstly I doubt many people are really going to do that either (do you leave a web site and then immediately post on your blog/Facebook page/Twitter feed that "Site X Sucks!!!11!", really?) and secondly even if they do then I suspect the odds of converting anyone they tell who acts on that information are very low.

No, it only assumes that you don't gain a positive net benefit from a networking effect in a network where people use ad blockers and then get upset and storm off your site if you do something about it. In the absence of data to the contrary, that doesn't seem like an unreasonable assumption.