At some point, it’s on you to know what your machine can and can’t handle. They put big letters on the front of each game telling you if it’s able to play on the series X and series S. It is right there lol. 
Also, with smart delivery, it would probably be trivial for Microsoft to have a modal pop up saying “this game is not optimized for series S and will not play, do you still want to purchase?”
No, the real issue here is developers (not their fault mind you). The moment Microsoft says “you don’t have to make it playable on the S,“ they simply won’t. Because why would you?