The issue is not the sharing of ideas, but sharing of how the software was made. It is more like saying: "if I buy your food, you have to give me the recipe or I will sue you." There is a huge difference between using gcc and knowing how it was created, for example.

Nobody is restricting you from creating a similar food or software (unless there is a patent, which is an entirely different issue). You have the right to create your version, and I have my right to sell only the product, not the process that made it.

Yet the maker of a software product can aid or hinder your ability to modify it by choosing to distribute as source or binary. That particular "right" requires no law.

I often wonder what RMS would do if he had the power to enact a law mandating that all software be free software - as a form of "positive liberty". Judging by his writings, I get the feeling he'd do it.

Wrong. Copyright is about "works", not "ideas".

My copyright of a work doesn't restrict your ability to copy the idea. In fact, this is what many Open Source projects do: they copy the idea or a proprietary software, but not the software (ie. the work) itself.