It's not even just the Steam thing, it's almost a bait-and-switch. The original Kickstarter promised a DRM-free game and at that time Steam was a "maybe". While they are technically fulfilling the reward of a DRM-free game it feels rather scummy to have left out "Oh by the way the DRM-free version won't be receiving future updates or DLC. For that you need Steam." Pretty sure a fair number of backers would have chosen NOT to back if that was in the original pitch. But it wasn't, it's only now after the developers collected all the money that that tidbit of info has come out.
The DRM-free copy is what people backed, and now it is the lesser version. According to comments I've seen the developer is putting together a FAQ that will be posted in a day or two to clear some things up, like will the DRM-free version support mods still and whether the DRM-free version will even be sold from here on out (the official preorder page only mentions Steam, so the DRM-free version may be a backer exclusive...which also wasn't mentioned or clarified in the original pitch). We'll see what happens.
Dunno why they aren't going with GOG. They are DRM-free and support DLC sales so whats the problem? The mod community can handle itself with the Nexus sites and such.
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Another point I've seen from certain backers is the idea that this is supposed to be different from just a preorder. They weren't just giving money to get a copy, they were giving money to help develop a game that others could also buy. So when the devs said the game would be DRM-free backers assumed that meant for everyone including non-backers. When the stretch goal for the 2nd city was put up and funded there was no mention that it was actually DLC and that backers would get it free while everyone else would pay extra. Many assumed that the stretch goal was to raise the money to make the game better in general, for everyone.
I think this is an interesting case for Kickstarter projects in general. The developers haven't technically done anything wrong and the backers will be getting what was explicitly stated, but it seems they were still mislead. It's like the genie in the magic lamp that technically grants your wish but it's not really what you asked for.