The calculation appears to be that given the chance, some website controllers will choose to trade confidentiality of public pages for better load times. In business terms, this seems a pretty straightfoward win in many cases, so I can see why some would sign up.
A publisher is free to switch to AMP, but choice needs to be given to the user to agree or leave the site the same way it happens with cookies. I wouldn’t opt in and now I cannot block Google tracking at the DNS level thanks to this.
We are, unfortunately, a long way from this being normal. Even as third parties doing things like running CDNs or doing TLS termination for other reasons has been pretty thoroughly normalized. Though offering it as a Firefox extension could be an interesting exercise.
I think publishers view AMP as a question of their sovereignty and choice. Since it's their website that's being potentially served by Google, it's their choice to make. There's absolutely a lot of room to dispute if this is the morally correct stance, but I also think it's not wildly out of line with other questions publishers weigh in choosing what they serve and how.
How do signed exchanges break blocking Google tracking at the DNS level? You already need to have google.com unblocked in order to get a results page that serves an exchange from Google.