This is a popular refrain from folks hosting read-only sites.
The problem is not "your part", it's the "between you and the client" part.
It becomes trivial to inject extra content, malicious JavaScript, adverts etc into the flow. And this isn't "targetted" at your site, its simply applied to all insecure sites.
TLS is not about restricting your ability to broadcast information. It's about preserving your ability to guarentee that your reader reads what you wrote.
TLS is free and easy to implement. The only reason not to do it is laziness. You may see TLS as a violation of your principles- but I see it as an attitude of "I don't care about my readers safety - let someone inject malicious JavaScript (or worse) on my page, their security is not my problem".
(If the govt want to censor you they can do that via dns).
How do you protect your physical letters from being opened by unauthorized parties along the delivery chain? You don't for the most part because we have made it a very series crime to do that. We could have done the same to badly behaving ISPs. Instead browsers have chosen to make planned obsolescence a requirement for the web.
Making it a crime is very regional, and enforcement is basically non existent. But opening it is not the problem here.
The analogous problem would be letters opened and anthrax inserted. That doesn't (often) happen because mail is physical and hard to do at scale. (And the anthrax cant mine bitcoin.)
Given the ineffectiveness of current laws around ransomware, bonnets, phishing, identity theft, online scams etc, I don't think a law saying "don't do that" would be a solution.
And ISPs are (by far) not the only offenders here. Every public wifi would be an equally attractive attack point.