That would be a difficult law to enforce without declaring martial law. If smartphones are any indication, it's difficult to design a device that cannot be convinced to run unauthorized code.
You're saying something different. "can run unauthorized code" is different from "must run this closed-source software". Both could be true at the same time.
What good is your free, open, secure channel if your phone is also a running mandatory rootkit, containing a GPS tracker and keylogger? See also: Carrier IQ.
Here's an easy way: promote the use of a security cryptographic communications protocol. Fund development of a plugin for Windows. Advocate or even pass a law that banks, the IRS, and other organizations support it. Ignore requests that the code be published and/or available for other OSes. Ta-da! You've effectively discouraged use of free OSes.
Okay, it's still possible to have a system without proprietary software. Just much harder, and not worth the frustration to most people.
"If smartphones are any indication, it's difficult to design a device that cannot be convinced to run unauthorized code."
True, but you could require that the device have a little semi-autonomous widget attached that monitors the rest of the device, can't be controlled by software on the main device, and can't be removed or deactivated without seriously reducing functionality of the device.
The government would simply have to say "put this in your phones, and require devices on your network have it enabled, or you don't get FCC approval."
"""That would be a difficult law to enforce without declaring martial law."""
I seriously doubt that.
There are current laws in practice that would have needed martial law declared to be enforced (or even passed) a century of even a few decades ago.
And we're talking about "detain anyone for any period" and such laws, so in comparison the general public could care less about an "only run state-sponsored non Open Source software" law.
What good is your free, open, secure channel if your phone is also a running mandatory rootkit, containing a GPS tracker and keylogger? See also: Carrier IQ.