Is linking just polite, or is it a core value of journalism?

I stopped caring a long time ago what journalists do amongst each other. Real journalists, the ones that dig until they find the truth, even if it is politically inconvenient, have become so rare that the word "journalism" has lost its meaning.

All that matters to me is respect to your readers.

I consider any author who doesn't link to their sources untrustworthy. Doubly so if the author claims to be a journalist. It's like someone standing in front of you with their hands demonstratively behind their backs saying: "no, I'm not hiding anything, really, but I'm not showing you my hands because I'm a journalist". Gimme a fucking break.

If there has to be a debate whether not behaving like a little child is a core value of journalism, than journalism is truly dead.

surely good ethics for any sort of writing is to cite your sources; a link is simply the web-appropriate way to do that

I like and agree with the comparison of links to citations. And you cite credible sources that bring factual weight to your text, not third party news sites that aren't verified.