Oh, the memories. I haven't seen this metaphor fight in like 8 years now, since web annotation basically died. And here we finally are with a version of this debate that people actually care about, because there's actual money at stake. Finally my years of prep are ready to pay off!

First, I suggest everyone resist metaphors. Physical metaphors don't work here at all. For one thing, the "free content" may in the future decide that you can't load the content in the page until after the ads and trackers confirm that you have indeed loaded their content and executed it to some degree, which wrecks every physical metaphor you can come up with. Physical content is dead. Web content is code. It gets to do code-y things. No physical metaphor will keep up.

Next, this obviously produces an arms race. The first iterations of this will be stupid and client-side, because it's cheap. When that doesn't work, server-side things can be done so that at the very least, you must download the ad; that can be confirmed fairly well in a non-forgeable way. The ads will also run arbitrary bits of code in an attempt to verify that they've really been shown; the client side plugins will retaliate by simulating the relevant code and not actually showing the ad. With the current state of web technology, this would stabilize in a sort of middle ground, where the client-side correctly doesn't render the ads, but the requisite requests to convince the server that you can see the content would by necessity still leak tracking information in order to convince the servers to show it at all. You could try to do things like use client-side stuff to scramble those values and share them among users or something, but that's fairly easy to detect at scale.

Where that becomes particularly relevant though is that the clients at this point are becoming increasingly outright deceptive and duplicitious. Instead of passively failing to render ads, actively deceiving the remote systems opens a new door both legally and ethically. At some point after this the publishers have both a legal and ethical case that you are not merely "experiencing the content as you wish", but actively defrauding them.

This, of course, won't stop people from arguing they have a right not to be tracked or advertised at, which means that they will essentially be arguing in favor of a right to have their programs make fradulent requests in order to get this "free" content. Which is fine and dandy locally, but consider what that means globally... and in particular, at this point how do you argue that it's ethically wrong to put trackers and ads in content, and for that matter even spyware, if the "other side" is retaliating in kind? If you're not careful with the ethics here you surprisingly find that you end up arguing in favor of the very things you thought you were arguing against, and end up making a "good for me but not for thee" argument, which even if you do end up believing that is still not a very powerful or compelling ethical/legal position.

Pardon if I'm leaping about 4 steps further down the road than most people here are at... I worked this all out about 12 years ago. (It's a bit dated now and I cringe a bit at the writing style, as befits any writer linking to something they wrote 12 years ago, but you can see what I worked out at http://www.jerf.org/iri/blogbook/communication_ethics . Perhaps remarkably, it is dead on topic for this very debate... there's a lot of related issues here.)