holy **** it's Turey
Yep it's me. I'm currently working at IBM (my salary is like woah

) while taking classes. Still trying to finish up my degree, only got a bit left.
I am fairly sure this is bull****. Parallax may have dissolved, but unless I am very mistaken the rights were never released. GOG still sells D1+D2 for $10. There's a few other games on that site that are of questionable legality as well, notably Commander Keen.
They seem to be hosting shareware versions; Z66 is only the first mission, for example.
For all the people talking about legality, this is the right answer. I've only played Descent on the site, but it's very clearly the shareware version, which means it has always been free to legally distribute. A fair number of the games seem to be full versions, but the ones you've been talking about are probably shareware versions.
For those who don't remember/weren't around for the shareware era, shareware is a cut-down version of the game, sort of like a long demo, which is distributed for free, in order to spark interest and increase sales. It's perfectly legal to host in this manner.