Because it's available in some abandonware sites; so is it?
There really is no Abandonware on GOG. Even the free games are here from agreements with the publishers, so everything here is legit. Even the ones that are freeware are better here anyway. Take, for example, Utlima IV. It's got all kinds of goodies you won't find on "Abandonware" sites, and even Ultima Forever (the official website for Ultima IV downloads) now sends you here to get the game.
There is only one legitimate form of "Abandonware" and that is in situations in which the owner of the game has decided to announce he or she is willing to let the game be downloaded freely. I know of at least one such game in which the programmer stated on a newsgroup site that he would rather people download his game from wherever than to let his work disappear. These situations are few and far between though.
With that said, there are many excellent games that are available free and legally including all the ones here on GOG as well as Elder Scrolls II, a couple of the Grand Theft Auto games and other wonderful legal freebies if you look enough.