GOG.com is simply to be commended since it is an ABSOLUTELY NO DRM company.
I think there's not much left to say, they do a wonderful job and I eagerly paid them for a couple games that I so badly needed to be "repackaged".
Summing up on the OP, one can also pirate any stuff and warez the hell out of the market after all, so why bother to pay ANYONE?
"Abandonware" has always sort of been a strange field, cause LEGALLY one could not download ANY copyrighted material unless the copyright owner writes that off and let it for free (as kkmic already said), thou a widely accepted exception was
"if it no longer commercialy sold, it is okay to download it", not a really clean one actually.
GOG capitalized on a powerful niche and does a STRENOUS job of looking for legally distribute copyrighted material no matter how "ancient", ready off the shelf for your newest platforms (DosBox runs also on non windows plaforms).
all in all a good deal for the player, IMO.
EDIT:
What bugs me is that GOG.com does not own the majority of the games they're selling (except for the Witcher series IIRC), and the copyright holders for a lot of the others are not the original creators of the game, meaning that you may end up paying a company that has nothing to do with the creation of the original gem you're buying.
It bugs me a bit too, but in that case the Copyright was either acquired by merger, bought from the original authors or got through a tender by foreclosure.
Hardly a GOG fault nor problem, and in any case it's only right you pay those right amid the law;
.....thou on a more philosophical wave....I also am a bit bothered about that, but I still think that its a good deal for the players.