Good points. It's a tricky area. I have an article underway (that I hope I'll get finished at some point - but you know how it is!) which is a longer more detailed version of the short form that you linked to (thank you).

I spent many years working in areas where plagiarism was an issue that had to be faced head on, so I got to know a lot about it.

Bottom line: plagiarism is theft - theft is a crime.

That said, two people can (and often do) write about the same topic and cover the same ground - that's not plagiarism if they're not copying each other's work.

However, if you summarise another person's work, you are getting into muddy waters. Yes, the summary is your work but you have relied heavily on theirs. There is room to argue the plagiarism case but I agree with you that in this circumstance, the original writer should always be acknowledged.