You're correct in that Steam doesn't mandate DRM for games sold on it. But it has two distinct problems:
1. Steam doesn't indicate on a game's store page if said game is DRM-free. You have to search for this information elsewhere and hope someone has the correct info. Sometime Steam will indicate if a game uses extra DRM apart from regular Steam-DRM, but I've never seen it advertise a game as DRM-free unless the developer/publisher has specifically put that info somewhere in the game's description. However, everything from GOG is DRM-free so they don't have to say anything because it's already a given.
2. Steam mandates use of a client. Even if you want to run the game outside of the client, you need the Steam client for downloading as well as updates. GOG has a client but you can easily download everything from a game's landing page once purchased. You then have an installer you can backup and the game runs via your regular desktop/Start menu shortcut. Much cleaner I think as I can then download and organize files in the way I want without dealing with the problems that can arise by using an unnecessary client.
What pisses me off the most about Steam is that its DRM is NOT EVEN NECESSARY. It doesn't stop pirating - I downloaded BioShock Infinite and Episode 2 (which contained the previous episode, all other DLC and the latest patch), installed them, copied over the modified steam_api files and ran it perfectly fine (even quicker than steam, since no client needs to be launched first and there's no "preparing" dialog window beforehand). It doesn't stop jack shit in terms of pirating, yet it's there because archaic publishers still believe that some protection is better than nothing even if that DRM has the potential to hider paying customers.