F4LL0UT: My thought exactly. Although I'm always kinda surprised when I read this stuff about donated games and extra content. It's usually easy enough to find high quality downloads for these kinds of things online and one would think that it doesn't matter to GOG or their partner if their source is an original physical copy or a download hosted by a pirate site.
It makes sense to have a source that is guaranteed to be original, though, if only for verification. As GOG.com is going to resell the software, it would be a big liability to suddenly discover that they redistributed some undesirable code modifcation, too. On the other hand, I remember that there was a case where fragments of a scene NoCD were found in a GOG.com game, so I guess they do if they have no other option.