Well, true. However, assuming good intentions, there are laws against working two/three jobs in the world in a manner that would rob you of your time to rest, which was presumably put in place to prevent a lot of undesirable things from happening. So far, I had no problems with side jobs as long as I agreed with my employer and made sure that my work/rest times are not in danger (nobody ever said no).

Yes. My attitude is that I've entered into a commercial exchange with my employer. They pay me money and in exchange get a set amount of my undivided time and attention.

They have no legitimate claim over what I do during the time that I'm not being paid. I may contractually agree to certain narrow restrictions (such as not talking about the company or the details of my work), but generally, my time is my time to spend as I see fit. In my worldview, any other stance is my employer making some sort of ownership claim over my life. That's unacceptable to me.

If what I do in my off time adversely impacts my performance on the job, then my employer can reprimand or fire me for poor performance.