I could work two different retail jobs in a day if the schedules work out. I cannot work 40 hours a week as an engineer, much less take on a second job (fortunately I can always find a few mindless meetings to make up my job). So long as I'm expected to work 40 hour weeks my company is justified in asking me not work a second job in my field as I couldn't anyway.

Now I could go out and get a retail job for after my regular job.