It is kind of crazy, but it's necessary. Nuisance plagiarism lawsuits can cost writers and publishers a great deal of money, time, and work, so for our own protection we need to take a zero-tolerance policy toward reading unsolicited story ideas. It's the same reason you need an agent to submit a spec script to a TV series, or why the open submission policy practiced by TNG, DS9, and VGR required you to sign a release form before submitting anything. Pro writers participate in the Trek Lit forum, so story ideas have to be prohibited, because otherwise we'd have to avoid posting here altogether. You're free to discuss story ideas in the Fan Fiction forum, however, because the pros know to avoid that one.
Problem is Trek Lit authors participate in the entire rest of the forums where you are allowed to post story ideas. Just look at the Trek XI forum and all those countless of alternatives and sequel ideas.