1st of all: yes, the desktop app is open source and follows a „pay what you want“ idea (including the option to pay nothing at all)
2nd: the mobile apps are open core. Means the core (the encryption library) is open source, but the complete app is not and you’ll have to either create your own app or buy the one Cryptomator has built for you.
Please read this to answer your question why these apps are not free.