The developers can choose. Many games are “DRM” in scare quotes, it’s not that developers are enabling this but they’re hooking into features such as cloud saves or the workshop and to get them running outside of steam you need to provide a steam.dll with stubs for some functions so the game doesn’t get confused.

DRM, at least from a programmer’s perspective, only starts once you actually a) check for integrity of game files etc. and b) check for integrity of that integrity checking code. The bulk of steam games will throw some error when you try to run them outside of steam, but they’re not taking any measures to prevent you from making them think that steam is running.

From a developer’s perspective – honestly, I don’t care. If a game gets published on multiple stores I’d generally try and make all of them the exact same version so the game will check whether steam.dll is available, use it if it’s there, and not if it’s not. If it’s only published on steam I may blindly assume that steam.dll is there and error out if it’s not because I didn’t bother to make a version of the main menu that doesn’t have a “workshop” menu item.

I don’t really mind whether you pirate my game, unless you’re a millionaire that is at that point I will judge your character quite harshly. But I’m also not going to spend time and effort on making the game easier to pirate.