I don't think copyright would have anything to do with it, since I don't think you can claim a legitimate copyright on an idea from a BBS post or blog comment. Any plagiarism suits that arose from such an incident would almost certainly be thrown out of court. The issue is that even frivolous lawsuits cost a lot of money to fight off. That could perhaps be a factor; screenwriters and producers make a lot more money than novelists. Still, Hollywood studios and production companies go to considerable lengths to guard against plagiarism suits, so I doubt someone like Orci would really expose himself to that kind of risk.
I haven't been following those comments and I'm not aware if there's really anything in them that constitutes a story idea per se. Something like "I want to see Khan" or "I want to see a followup on the Vulcan refugees" is not a story idea, because there's no plot there, no events or character arcs being described. It could just be that nothing in those posts has actually crossed the line into a story idea.