If there is no way to legally buy the game, then I consider it abandonware. Even if I were to buy a used copy, none of the money would go to anybody who made the game. For example, quite a few early Sonic games are available on Steam, directly from Sega. They're not abandonware because the current owners of Sonic are still making money from a legal purchase. Same story with things like the Virtual Console, XBLA or PSN.
On the other hand, as far as I know, there is no way to buy A Mind Forever Voyaging, either a download or a boxed physical copy. Nobody owns the rights to it, or else they're sitting on the copyright with the intent of never releasing it. If I could buy this game anywhere, then somebody please let me know. I would love to support anybody who would still sell a text adventure game, especially one as unusual as AMFV.
One caveat to my outlook is that the availability of used copies influences my decision to buy a copy or download. If there's a handful of copies of (insert really rare game) for $90 USD on eBay and that's all I can find, I'll definitely download because that's overcharging from somebody who has no link to the publishers nor developers. If I can find a $10 copy at some used vidya store though, I'll buy it. One reason to buy used copies is that a legit copy on the original hardware is always better than emulation. Another reason is to support physical used game stores, which in a lot of cases is the only way to get some new-but-not-on-store-shelves games.
Is it legal? No. Is it fair? The way I do it, I think so.
That said, check out Leather Goddesses of Phobos, A Mind Forever Voyaging and the Spellcasting series. All of them are excellent adventure games written by Steve Meretzky. They blow any "____ Quest" game by Sierra completely out of the water, both in terms of writing and in puzzle quality. If Infocom had made a smoother leap into the graphical era, then Sierra would have just been a small, unfunny blip on the radar in comparison.