A year or two prior to this, they were also putting linked sites into iframes, with the "digg bar" on top. So they had managed to alienate not just their users, but also the site owners that all the content linked to.
Actually it would be nice if this was a browser feature. Click on a link on a site like HN or Reddit, browse to the new page as usual, but you have a little inconspicuous indicator in the toolbar that lets you vote or comment on the origin site easily.
The selfishness of it is obvious if you think through what would happen if someone posted one of these dig iframed urls to another digg-like site, which was then itself posted to a third digg-like site. 3 stacked headers, yay!
I agree that site owners shouldn't like this behavior. Still, we've seen cases where content owners have welcomed things against their interest in the name of engagement or hype.
For example, a lot of video producers started putting content on Facebook (where they received no revenue) because they were getting lots of views on Facebook compared to their own site. This eventually left them without the revenue they needed and they dwindled.
Yes, site owners shouldn't like these kinds of things, but we've seen sites chase a lot of crazy trends. The Oatmeal has a cartoon that kinda sums up some of this: https://theoatmeal.com/comics/reaching_people.
Even today, you'd think brands would want to move to ActivityPub where they could run their own server and actually have control; you'd think influencers would want to move to ActivityPub where they wouldn't be beholden to Facebook looking for money to boost their reach or Musk's arbitrary moods. Instead, so many are sticking around on platforms they know are looking to gain an advantage over them. I'm not even suggesting abandoning those platforms, but cross-posting to ActivityPub would mean building a future you're more in control of.
There's a lot of platform behaviors that aren't good for sites that many sites end up being enthusiastic about.