Check Home of the Underdogs - they explain everything.
Though IIRC, Descent I is not among their downloads - then again I could be wrong.
As for the legal turmoil - if HOTU has it, even if it's not legal you're safe to download and use it. HOTU has become quite popular, and are keen not to get in trial with any company, so whatever they have for download is pretty much as abandoned as a software can be - even freeware in several cases.
Recently thanks to several films (Bethesda - Terminator games) and "special-releases" (Hmm...Intergay 20 ect..Hmm) they had to remove several titles.
Extreme Assault is up for download! (Last time I checked). That's another classic from the action-arcade-3d-shooter genre.
It has better graphics than Descent 1/2 even if it's not 360 degrees stomach wrenching action.