There's also another meaning of freedom: freedom from having to care of worry about the platform, and being free to create, because you have a thing that "just works".
For musicians that's more important than "route everything to everything" (for which there are solutions even for commercial DAWs, e.g. Rewire for those that do care for it).
It's all about tradeoffs, but Jack and "freedom to modify DAW code" is pretty down on the list of priorities for 99% of musicians.
Also, a lot of time using FOSS is a choice we make not because of the price or principals but for convenience. I simply don't want to juggle licenses and dongles between home and studio computers, or for that matter be forced to install licensing software that is more or less a rootkit...