I've rarely noticed that. Are you saying someone will copy both the question and the answer from a place like experts exchange or a blog? Do they post them under different accounts to make it seem less obvious? Or do you mean the semi-rare occasion where someone cites a blog post?

I think I've seen an unsourced copy only once, I think it was a new answer where nobody else had seen it yet and I edited in a link to the source. But this is many years ago, I don't remember the situation exactly (and I visit stackexchange sites a lot, more than most people I think, since it's the only social site where I permit myself to go to during work hours).