I agree with what everyone has said, but in reality, it would be incredibly rare to find an employer who would attempt to take things that far, because they would never be able to keep any staff. Even people desperate for a job won't stay there any longer than they have to stay...they would be looking for something else.
What most employers will try to restrict people from doing are things that would raise health insurance costs, or things that have the potential to interfere with the employer's business. If an employer is trying to restrict someone from working a second job, its likely to be because the employee is tired and not alert enough at work, or the employer needs the employee to be completely flexible with hours.