Niggles: you do have to wonder why they didnt just self publish and how much control publishers have if devs cant even control the pricing? (wonder at relations between gog and kalypso.....)
I'm guessing that they found it easier to just sign up with a publisher who's already on Steam, rather than negotiating everything from the starting point. It has its obvious downsides (as can be seen), but it presumably makes their work otherwise easier.
I think it was mentioned somewhere that Cinemaware being on GOG as a publisher was thanks to fan activity. Otherwise they probably would use some other publisher, like with Wings remake.
Of course having different publishers depending on the title and store makes everything very complicated, as is obvious from the pricing gap between GOG and Steam, the apparently impossible-to-get backer keys, etc.
I can only assume that the profit margin from Steam, even under a different publisher, is relatively so big that it justifies losing full control of their own games.