Because I am pro-capitalism, I utterly disagree with your premise. In a real contract, parties can negotiate and come to a meeting of the minds. Here's how it actually works:
* A website serves me a page with a place to put ads on it.
* I reject their offer to serve me ads.
* The site has the option of deciding not to serve me any more content, typically by showing me an anti-ad-blocker popup. If they continue to serve me, they've agreed to my proposed contract alterations.
* If they choose not to serve me, I can decide to accept their final offer (by disabling my ad-blocker) or reject it (by closing the tab).
What on earth makes you think that the negotiation ends with the initial offer? That's not how bargaining works. This isn't some Soviet-style take-it-or-leave-it scenario.
Is buying milk at your local supermarket a Soviet-style take-it-or-leave-it scenario?
If not, at what point during your milk purchase does the negotiation step that you hold to be important for capitalism take place?
I put it to you that take-it-or-leave-it-ness is orthogonal to the capitalism-socialism axis, and that the take-it-or-leave-it nature of viewing an ad-supported website is no more socialist (and no more alarming) than buying milk.
Regarding "negotiation":
> * The site has the option of deciding not to serve me any more content, typically by showing me an anti-ad-blocker popup.
Are you indeed claiming that today's ad blockers operate by explicitly rejecting a request sent from the main site as part of some standard ad negotiation protocol? Because if so, I would agree that this amounts to a negotiation with the website as you say.
But this would certainly be news to me. It must be a recent change, since for most of my life, ads have simply been hyperlinked images/objects/videos/IFrames, or sometimes inline text generated server-side or on the client using JS, and the only mechanisms available to implement ad blocking were implicit, and based on subterfuge: By preventing fetching of that content in the first place (in a variety of ways), or by fetching it but then hiding/obscuring the result in some way. None of which amount to "negotiation", obviously.