gameon: My guess is that it's drm free here, and perhaps the amazon one is for the steam version? I'm not sure whether amazon does drm free games. But at only $2.99 even with steam, it's worth buying in my opinion.
Coelocanth: It's not Steam on Amazon or they'd list it at the bottom of the specs/system requirements. I think Amazon games that don't list DRM are DRM-free, only needing the Amazon client to DL (you can use the same sort of trick as with GG to get the files DRM-free). That's my understanding, anyway.
Some games are labeled DRM-free, like Divinity 2.
Not sure what it means when they aren't labeled with anything.