Nonono, I didn’t mean to say that the FSF requires read-only hardware. Quite the opposite actually.
The FSF demands that you can modify the software. I.e. that you get the sources, that you get the tools required to build them, and whatever else it takes to actually build it after modifying it. And that you can redistribute the new sources.
My point is: For the mission of Purism, it would be “just enough” if all software were “shared source”, i.e. if you can read it. In that situation, you could still verify that the software does nothing that restricts the user, and that’s the mission of Purism.
Now I’m evading the paddle by saying that FSF-approved would still be better; it’s just not (currently) practical.
Another point why one could ignore the FSF: It disapproves of all distros that merely allow the user to choose non-free software. However, that would be a non-issue for Purism: Just don’t install the non-free repositories. And, maybe, actively look for free alternatives. Though that’s hard to do in a world of patented Videocodecs - a distro that cannot play videos may be free, but it’s not useful for all tasks.
Here again a non-free codec would be against the FSF’s endorsement, but it would fit with Purism: Just code the thing up, cough up the licensing fees, publish the sources, people can validate the sources and experiment with them. They could even modify the code for experiments - they wouldn’t be allowed to publish the modified code, but they could publish the patches.