The package repository is full of proprietary software, they only use the proprietary version of Linux and they default to installing/including the proprietary peripheral software for wireless hardware without asking.
Images for a handful of devices (i.e. certain switches) appear to be free by accident, as the Linux .config used doesn't compile the proprietary parts in and proprietary packages don't seem to be installed by default - but I'm not certain.
Of course, people have released a fixed version with GNU Linux-libre and no nonfree packages for some devices; https://librecmc.org/