Response 1: whether this question already has a technical answer that you find satisfying does not affect whether using an ad blocker is theft.
Response 2: a few sites are already popping up messages asking you to turn off your ad blocker, which is at least a partial existence proof.
Response 3: if you want to know whether I got your ads, use cookies or make the request url unique to me, and then check that your ad servers served the appropriate requests before allowing your content servers to serve me content.
But seriously, I shouldn't be required to solve all these problems for you as a user in order to not be called a thief. If you want a more explicit contract, then require a log in. If that adds too much friction, invent a better a better solution.
*Edited to remove the inflammatory suggestion to go out of business if you refuse to negotiate.
I edited my response because I realized it was inflammatory. But to be clear, my viewpoint is that the client/server and viewer/publisher relationship is a negotiation. The publisher should make content available under terms they find acceptable, and the user should be free to refuse to view things that they find unacceptable.
I don't think it's right that publishers seem to view me showing up at their front door as carte blanche to abuse my network and system resources to the full extent that JavaScript allows. And I think it's wrong to characterize user pushback as theft. Other kinds of "theft" are criminal behavior. Pick better terms.
That's actually how all businesses work. If the terms of business aren't acceptable to your users, you lose the users and your business will fail.
Adverts are non-consensual hijacking of users resources, so you could argue that advertisers are committing theft. You're welcome to use a paywall on your site, or hide content from people using adblockers if you want to negotiate from your side.
That's pretty much the sum of it. And frankly, there's nothing wrong with that. It's how markets work. You provide something of value, and someone purchases it, or you provide something of no value, and the market refuses to buy it.
Response 2: a few sites are already popping up messages asking you to turn off your ad blocker, which is at least a partial existence proof.
Response 3: if you want to know whether I got your ads, use cookies or make the request url unique to me, and then check that your ad servers served the appropriate requests before allowing your content servers to serve me content.
But seriously, I shouldn't be required to solve all these problems for you as a user in order to not be called a thief. If you want a more explicit contract, then require a log in. If that adds too much friction, invent a better a better solution.
*Edited to remove the inflammatory suggestion to go out of business if you refuse to negotiate.