There's no GPL in the BSD sources used by Apple or Sony. They are free to release their operating systems as closed source; Sony does this. Apple releases Darwin sources "out of the goodness of their hearts", meaning, back in the 2000s they wanted to capture mindshare amongst the tech community for whom Linux was the strongest contender. Now that the future has refused to change, the year of the Linux desktop never materialized, and macOS has become the default developer's workstation OS, Apple has been much more sparing with Darwin source drops and may cease them altogether.