hmm, if you order from grimrock.net directly, you get both... DRM-Free Standalone and a Steam Key... on steam, you will only get the Steam Version and on GOG, you do only get the DRM-Free Version at the same price tag.
Does not sound fair to me. (Pre-Ordered here anyway to support GOG)
That's fair. They want as much exposure as possible, so they sell on GOG and Steam. They also want as many of their sales as possible to be directly from them, since that gives them more revenue per sale, so they make direct sales as compelling as they can.
Most buyers aren't going to look at the dev's website anyway, so selling on GOG and Steam does nothing but benefit the devs. The aggregators earn a profit and the exposure will probably increase direct sales rather than cannibalising them.
Humble Store + Steam is definitely the best deal, but I can understand if some people prefer to consolidate their game collection here. Of course Steam has no advantage over Humble (which comes with a Steam key) but a huge chunk of their potential audience will find them on Steam and not bother shopping around.
The way it works now is a win-win-win-win situation. GOG has an uphill battle with all this competition, but they do get some sales in for relatively little investment (they don't need to reverse-engineer this like they do for their older games).