[tambre]

Usage of Steam's DRM is actually optional. There are plenty of games that you can copy the files of and run on another system without Steam present. [0]

[0] https://www.pcgamingwiki.com/wiki/The_big_list_of_DRM-free_g...

[immibis]

Optional for the developer. Mandatory for the player. Although I heard rumours it's easy enough to install alternate Steam DLLs that always return "yes" to "is this game licensed?"

[Sakos]

It's trivial. You replace a dll and the Steam API check is circumvented.