@p >The issue with proprietary software is that you cannot modify it: you cannot make it do what you want, and you are beholden to an external authority that controls the software and dictates acceptable use
That is one issue of proprietary software, but there is many more.

>Cash is no different.
Although the government may attempt to restrict how cash is used, they have issues enforcing such in practice - as you can send only so many glowers to physically carry out the restrictions.

Meanwhile, proprietary software is usually self-restricting due to the absence of source code and impracticability of performing anything but trivial changes, due to the requirement of how reverse engineering and then patching the binaries is required to make any modification - with the result being evidence of copyright infringement performed, very much unlike a discrete cash transaction you made with your buddy that nobody else knows about.