On HN, nobody signs over any IP, nor does the copyright holder necessarily grant a license of any sort. (If you doubt that, consider the fact that I can submit anyone's URL to HN. So submitters aren't assumed to have any authority over the IP.)
With periodicals, though, there usually is a contract with writers where rights are assigned to the publisher. Maybe the copyright, maybe just a license. There's usually something along those lines.
So the difference between HN and such periodicals (which may or may not include HuffPo) is the difference between merely sharing a link and signing over IP rights.