It's because in the US the theory is that by hiring you salaried, they hire your mind and creative output, not a set labor. This means they pay you for anything

It's because in the US the theory is that by hiring you salaried, they hire your mind and creative output, not a set labor. This means they pay you for anything your mind produces and then try to claim ownership for anything your mind produces whether you were in-office or not. (this is usually not enforcable unless you actively build competing services)