Most employment agreements are such that the company owns it even if it is outside of normal hours. So inventions you come up with on your own time are not yours.
I guess my gripe is that most companies expect a blurring of your work/personal time when it is in their favor. It is far to common for a boss to call someone at home, or expect work to occur remotely after hours or on weekends. So morally, the opposite should be true.
Salaried positions do NOT require 8 hours of work, they can't legally. It is the flipside of the no-overtime equation. You have to be paid for days you work, but you are paid to do a job, not work a set number of hours. It gets really fuzzy (usually not in a workers favor), but essentially salaried workers are supposed to have a certain amount of autonomy in how they carry out their work.
In days of yore companies like HP, and Google (somewhat laughably) encouraged outside projects with a notion of 10% of your time being an acceptable amount to spend on non-sanctioned fun projects. Many side and home projects turned into major revenue for the company, or a new business. It was viewed as a good thing. It has become much more restricted and legalistic these days.