You don't have to assume people are going to be bad, but it's reasonable and prudent to expect it from people who have already shown themselves to be so (in this context).

I trust people until they give me cause to do otherwise.

Training on personal data people thought was going to remain private vs. stuff out in public view (copyright or not), are two different magnitudes of ethics breaches. Opt OUT instead of Opt IN for this is CRAZY in my opinion. I hope that the reddit post is WRONG on that detail but I seriously doubt it.

I asked Claude: "If a company has a privacy policy and says they will not train on your data and then decides to change the policy in order "to make the models better for everyone." What should the terms be?"

The model suggests in the first paragraph or so EXPLICIT OPT IN. Not Opt OUT

No, nbulka is correct. People should not shrug off and accept things that are wrong just because it's to be expected. It's one of the worst things you can do because as already pointed out, it just normalizes wrong.