Blackdrazon: While Zork 1-3 were never released freeware despite internet rumours, Zork: The Undiscovered Underground definitely was released for free. I wouldn't mind it as a GOG freebie or as a bonus on the Zork Anthology, but there's really no reason not to grab an interpreter and play the game right now!
Actually, Zork I-III were temporarily released for free by Activision to promote ZGI both on the Internet and on various promo CDs packaged with game magazines much in the way EA has released Ultima IV for free to promote other Ultima products. The Zorks were only downloadable from Activision's site at that time. Activision has also allowed the iPhone program Frotz to run the Zork games as a promotion for the Lost Treasures set.
Other games such as Red Baron and Betrayal at Krondor have seen limited free release by the publishers to promote other products. The strangest of these was Betrayal at Krondor. I have the game not from a computer store, but from having the CD bound into the hardcover novel of the first game. It's a great game, and a good read as well.
So it's true the Zorks were never released as unlimited freeware but as exclusive free products. It's pretty much the same as the Ultima free games (IV and the Worlds) which are free to download but are exclusively licensed by the publisher to GOG.