Steam does some kind of authorization checks to restrict your ability to do certain things. I'm sorry if I'm being vague, but the exact workings behind Steam's digital restrictions are not usually discussed. The most thorough explanation I could find for how Steam's digital restriction mechanisms work is, "You are required to use Steam to launch your games and Steam is responsible for making sure you are authorised to play the game." Exactly what Steam does to this end might even be a secret, for all I know.
These features depend on Steam being defective by design. So any libre client that can replace the proprietary Steam client completely would undermine them. Valve isn't going to do this, not only because they want those digital restriction features, but also because it would anger proprietary game developers wishing to take advantage of these features.
I don't know why you think it makes a difference if Valve undermines the digital restrictions with an API instead of a client, or alternatively, why you think an API which is incapable of installing a large number of games would be something Valve would bother with.