DU is a hellish place. I don't know about this Skinner (the head of DU) business at all. If you post on DU, then your posts would be public and I can't imagine how that would be a legal issue. I'll say this, their thinking is vile and hate filled. Keep exposing them.
I'm not a lawyer, but I've been following the legal issues surrounding this sort of thing for years, and I can tell you that there's not a damn thing Skinner can do about this. Posts from one discussion board are fair game to be posted on another discussion board, period. The only thing that would alter this reality would be if PJ-Comix was somehow personally profiting from the use of the posts (for example, by collecting DU posts into a book and selling them).
Sure, Skinner can get a lawyer to write a threatening letter, but that's just a scare tactic.
I should add to my above post: Presuming this is from the real Skinner in the first place, which I don't think it is.
Posts on public forums are, well, public. I remember the very first public post I ever made on the Usenet back in '96. Talk about nervous! I made my post and cringed at the thought that it was out there for all to see. Of course now I take it for granted but it is PUBLIC. Somehow you can't put the genie back in the bottle once it is out there.