I am flabbergasted. How self-absorbed does one have to be to be offended by someone re-using sections of a paper one has done earlier in a later paper?

No, seriously. If I write 5.000 words on a topic and then re-use 500 words I myself have written earlier without attribution because they are appropriate to the current article, how do you make that «bad manners».

I meant what I said about people who can’t see this without help earlier.

I think it depends. The popular exposure to this idea, where you can be accused of self-plagiarism for a paper you write for a class, does seem stupid, because obviously your prof hasn't read your paper you wrote in another class and you're not 'wasting' anyone's time.

I can also appreciate that in a "publishing papers as research" context you're completely right.

That makes no sense, either people don't know about the previous work and thus it has clear value. Or they do and they can easily skip it. Beside for a lot of work it be great if you could just literally copy and paste fragments if your previous work to deepen out some reasoning.