That's a very interesting & useful devil's-advocate argument.

I think your own answer is the most sensible one: the freedom doesn't come from the bits intrinsically, it comes from rights (and information) alongside the bits.  Computers runs bits, but humans edit source code.  Thus, a law saying "users can't run non-free software" wouldn't make sense anyway -- the way to legislate that would be to require that suppliers can't *supply* non-free software.  E.g., just as HIPAA law (in the U.S.) requires that your doctor give you the primary sources of your medical data when requested, including X-ray images and everything, a software freedom law would require software suppliers to supply source code.  The user wouldn't be the subject of the law, the supplier would.