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.

I would imagine that it's because they want you to pay for it. Generally speaking only drafts of the standards are available, and even those become locked down after the completed version is published. Your comment about C++ being open is incorrect: the C++17 standard is for sale on the ISO website, while the C++20 standard (not completed at the time of this writing) is available on GitHub. From the official webpage:

Q: Why are the C++ working materials freely available on GitHub when the standard must be purchased from ISO or another standards organization?

ISO holds the copyright for all balloted and published versions of the C++ International Standard (IS) and related C++ Technical Specifications (TSes). The GitHub repo contains the incremental in-progress working source snapshots as they are edited, which may frequently be in an inconsistent in-progress state as edits are applied, and which does not contain the final source for any published standards.