Why are the documents from the C Standard working group password protected?

The drafts are password protected when they contain only the material that has been or will potentially be published as the ISO standard itself. Because C17 was basically a bugfix release (no new material) this was unfortunately the case for most of the development phase of C17.

But as soon as we switched to "C2x mode" the drafts are open. The first of these, N2310, is still pretty close to C17, so this could be a good base to work with. (And maybe this will draw more attention to the ongoing work on C2x, enroll in your national standards body to participate!)

Whether or not this "belongs in the public domain" is certainly a matter of debade. ISO doesn't work like this, unfortunately, and there is not much that anybody can do. At this time, having their special international status, they own the copyright of the standard itself. But they do not own the work surrounding it, the papers that people present etc, so anybody who contributes might still get their credits.