This is actually an interesting part of the culture around copyright: People seem to think copyright law is actually about plagiarism, so as long as they explicitly say they don't own something they're in the clear. It doesn't work like that, but it's how people seem to want the law to work.
What's more, plagiarism is reliably punished using whatever social means the community has. As we see here, in fact: Mentioning it is pretty much the only recourse left, so that's what we do.