nijuu: Im curious. Why would/should GOG allow their version to be sold on another service ?
My, purely personal, theory
GOG actually made the original Deal with Hasbro/Interplay estate/whoever had still had to bring abort for a re-release. That was around 2009/10 when nobody really thought old games could sell and even indies weren't as big as now. The whole industry was looking "COD/ME/DA", you know, bigger budget, high profile voice actors, better graphics in a nutshell, the whole "Hollywood action movie next gen package".
Therefore GOG was, in being dedicated to this series and the negotiation, possible to strike such a deal in the first place, simply because nobody bother with "ten year old games". Part of the deal with Atari was (again speculation on my side), that GOG got a one year exclusive deal on the D&D games we see here ( a lot is pointing in that direction). But GOG had to provide there mastered builds to Atari for an eventual rerelease (Atari probably wanted to test the waters first, try to sell "reinvesting in 10 year old outdated games" to your board of directors.). A simple "quid pro qou": negotiation and legal/technical groundwork for one year of exclusive distribution rights (and/or a bigger than usual cut for each game sold).
Seeing how this impacted on the gaming community and catapulted GOG in the top (or at least higher) tier of digital distributors, Atari (being out of touch as most big time devs and publishers) realized the goldmine they had on their hands and started selling the game to other retailers. Why they didn't go to Steam first? No clue, maybe they didn't want to compete with their own boxed version to much (at least on the american market).
Imo, the only reason GOG was able to pull this (master) stunt off, was because the industry (and especially Atari) was so out of touch with what gamers wanted and that they would gladly buy it
again for all the services that GOG offers, that they didn't think giving one year of exclusive rights was such a big thing. (Atari exec: "Wow, exclusive rights for a game nobody will buy anyway, sure GOG/CDP, you can have those. ...Idiots...")
CDP and Atari had dealings before (Witcher publishing), so maybe there are some factors that I'm missing which affect other games. GOGs most important gain was the PR, not the game sales themselves.