a) That's speculative, the E.T. and others have never confirmed there are licensing issues, though they haven't denied it either. There are very few native linux games from big publishers with whom licensing would be a complicated mess, and dosbox games without the windows installer are not necessarily for another platform.
I believe E.T. did say something after it was mentioned as to not really knowing if they did have such rights. I would actually be surprised if they didn't have to renegotiate anything existing. It annoying that different platform versions are being segregated as separate products, but they seem to be in a lot of cases. If I had to guess its because some devs are having to pay other companies to help port their games, which ends up giving those people some rights that have to be dealt with.
I'm pretty sure that's one major hurdle to overcome. I like to point to Shogo: MAD as a good example. The IP and rights to distribute the Windows version of the game are owned by Monolith. However, the Linux port was done by Hyperion and published by Titan Computer. So who owns the rights to the Linux version of Shogo? We're lucky that both companies are still around, so we'd have to do negotiations with them; we can't safely just assume that the rights would revert back to Monolith. And then there's the issue of support. The Linux port of MAD was released in 2000, and there's no guarantee it would still run on modern distros. And what distros do you target? Which versions of that distro?
The same holds true for a lot of other classic games on the GOG catalog as well which have had native ports for Linux released in the past.
The simple truth of the matter is that while yes, there are people who would buy Linux ports of GOG titles, the question is whether there would be enough of them to make all of that effort profitable (relative to the cost), and right now I simply don't think that the numbers support it. You can't make an immediate comparison to the HIB because all of the developers own the rights to their IP and all of their OS distributions, and they're the ones handling support. For GOG, all of that would essentially fall on them.
I hate to say it, but that applies doubly so for Mac users, since the proportion of Mac users who'd buy games from GOG is (I'd wager) a lot less than the proportion of Linux users who'd buy games from GOG, and some significant Mac porting and publishing companies have long been out of business.