Public Domain (e.g. SQLite) is the freeest possible license. Anybody can do literally anything with it. Even things that might infringe on some future freedoms, like not ensuring source code versions of modification are available. The freedoms to make unfree, i.e. not right, choices is strictly more freedom than the GPL. The GPL tries to make it so that the only choice is to do the right thing, but in a set of choices (i.e. freedoms), the right thing is strictly a subset of all possible freedoms.
Stallman wants, real hard, for this restriction of freedoms to be viewed as somehow "more free", and is fairly hardline about it. I wish he'd stop using freedom as his motivator and just talk about ensuring information access, which is really what his goal is.
The usual reductio ad absurdum is: In another world, you are allowed to sell yourself into slavery. In this one, you are not. How can the world containing slaves be the one that's more free?
In other words, the code is free until it is added to, becomes proprietary and - if a network effect adheres - then that proprietary version can dominate and in effect extinguish the public domain version. Certainly extinguish nearly all its value. The once-again-enslaved code lives and thrives, the freeman code perishes, or nearly so. That's a good argument, but the whole problem is created by an IP system that perverts the concept behind copyright. Paintings don't prevent other paintings from competing, code with a network effect or lock in does. Copyright should either never have been applied to code, or be for a very short period. Let's say, four years or eight years. Then BSDL code going proprietary isn't a threat, it's a bonus: more free code soon enough.
Stallman wants, real hard, for this restriction of freedoms to be viewed as somehow "more free", and is fairly hardline about it. I wish he'd stop using freedom as his motivator and just talk about ensuring information access, which is really what his goal is.