> the browser could easily warn you with a big "insecure!" page when you click on the button.

But this implies that whenever you click on any link on a page served via HTTP, the browser should warn you with a big "insecure!" page. I think this is far more obtrusive than a simple "not secure" banner next to the URL.

> I don't think https fixes that even for non-sketchy websites

You're right, if the website served over HTTPS injects random JavaScript/is poorly designed. However, the danger with HTTP is that every single website is vulnerable to this attack, not simply the ones served by malicious or incompetent hosts.

The donation request may not always be in the form of a button. Sometimes it may be a request to make the donation by another means such as by sending bitcoins to a specific address.