The same morons who think that re-using something you have written before in academic work without quoting yourself is (self-)plagiarism for which you should be sanctioned?
(Yes, they are morons because no reasonable person would think this is fair. You need convoluted nonsense arguments to justify this)
If I sell a novel to Simon & Shuster, and assign the copyright to them as part of the deal, I can't sell the same novel to HarperCollins under the same terms. I can't sell my movie to Disney AND Sony.
This is the same, but the price is zero. You can take or leave the deal at that price, and you'll probably take it for sufficient academic prestige points ('impact factor', visibility, etc).
By the way, some publishers such as IEEE explicitly allow you to post preprints for free download, eg. at arXiv and on your own web site.
It's bad manners and a waste of people's time and attention to present previously published work as novel.
Repeating a phrase or two in a document's introduction isn't going to raise flags from any serious people, but copying data, analysis, or large swaths of text? That's a paddlin'.
plagiarism, with heavy sanctions, of self is of course ridiculous, but having as a standard that you should cite yourself when doing it is not a bad standard. As a reader, it might trigger a "where have I read this before" reaction which is akin to confusion; also having notice that there is another paper on this topic could be quite useful.
Because if not, people will game the system and submit the same paper to dozens of journals at the same time, and journals are for publishing new, original work. And because in a class, you are being evaluated based on your ability to produce new, original work.
Do you also get angry at a track and field meet, when you show up with a motorbike and get kicked out for violating the rules?
But I don't expect someone who writes that much of an aggressively pompous yet uninformed statement to understand. Chesterton's Fence much?