What rights do BSD contributors lose? All the community code exists, the community can continue without the commercial changes, the community is not required to use some commercial fork. They lose nothing if some contributor chooses not to give back.
They lose the rights to take advantage of the improvements that the commercial contributor has done to their code, while the commercial contributor does not lose the right to take advantage of the improvements that the free contributor has done. You may agree or disagree with this, but it is objectively a loss.
Furthermore, users of GPL'd code decide not to give back at times too. They can use some a commercial fork internally and benefit from community work and not give back.
The point is that with the GPL they cannot commercially fork code written by me. Of course they can do whatever they want with their own code.
Also, various commercial users of BSD code have a pretty good track record of contributing back.
This is irrelevant to the discussion. When people make laws against theft, they don't think about the fact that most people have a pretty good track record of not stealing. Laws (and contracts) must be written with the worst case in mind.
What rights do BSD users lose?
100% pragmatic example: GPLv3 bash has a serious bug (any reference to reality is purely intentional). GPLv3 users patch, recompile and they have lost no right. BSD-licensed phone firmware has a serious bug. Users lose the right to make use of the phone they bought and not be pwned by hackers while doing that.
The GPL does *not* offer greater freedom, it creates restrictions to force behaviors it believes benevolent. Forced benevolence may or may not be a good thing but it is not freedom.
I believe that my rights to own property and to live are freedom. They exists only because other people are "forced to benevolence", in particular not to steal my stuff or harm me. Try to convince me that this is not freedom.
Translated to the software world, can you argue that the ability to fix the code of a program that I use is not a freedom for me? I'm free from bugs. I'm free from hackers. I'm free to add new features. I'm free both in a practical and philosophical sense.