OFC you don't know what Zork and the ZMachine are about. Zork it's propietary, but the standard virtual machine isn't.

You have several interpreters to choose form: Frotz, Fizmo, DFrotz, ScummVM (yes, that one, albeit a simple terminal with Frotz it's much better), Lectrote...

Altough I'd call the original MIT Zork 'Schrodinger licensed', because today they are fighting back and forth with MIT veterans so the original source from Dungeon gets a libre compatible license instead of a non-commercial one.

BTW, a libre licensed ZMachine game, Tristam Island (and Calypso, too) will run on far more plataforms than Doom itself. From C64's, to smartphones, pens and even a Game Boy.