In some jurisdictions, they cannot be "public domain", unless their authors died at least 70 years ago, which I very much doubt. They are probably distributed under a so-called "permissive license" (aka "lax license", aka "pushover license"), which lets anyone do anything they want with the work. Including changing its license for a proprietary software license.

The other category of free software licenses are copylefted licenses, such as the GNU GPL. Using a copylefted license, your work cannot end up in proprietary software. It is the whole point of the copyleft: preventing the middleman from stripping out the freedoms you want to give to whoever uses your work, modified or not. See https://www.gnu.org/copyleft/

Now, if the programs you are referring to do not bear any license, they are "all rights reserves", hence proprietary, under the Berne convention. See https://www.infoworld.com/article/2615869/open-source-software/github-needs-to-take-open-source-seriously.html for instance. Here is an excerpt:

You don't have to include a copyright statement for your creative work to be under copyright. In any country that's a signatory to the Berne Convention, copyright -- or stronger -- is the default as soon as something is created. If you completely ignore the subject, all your work is copyrighted to you (or to your employer in many cases), and anyone who copies it to use or improve it is in breach of your copyright.

See https://www.gnu.org/licenses/license-list.html for a long list of licenses. For free software licenses, the description usually tells whether the license is permissive or copylefted. About "Public Domain", that page says:

If you want to release your work to the public domain, we encourage you to use formal tools to do so. We ask people who make small contributions to GNU to sign a disclaimer form; that's one solution. If you're working on a project that doesn't have formal contribution policies like that, CC0 is a good tool that anyone can use. It formally dedicates your work to the public domain, and provides a fallback license for cases where that is not legally possible.

And about CC0:

CC0 is a public domain dedication from Creative Commons. A work released under CC0 is dedicated to the public domain to the fullest extent permitted by law. If that is not possible for any reason, CC0 also provides a lax, permissive license as a fallback.

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