Does the employer have the right to ban a employee from having a side hustle?

[Min George]

A friend of mine that is working for an oil and gas company told me that staff working in their company are banned from having a side hustle.

Does the company have the right to ban a staff from having a side hustle?

 

[eLdavis]

I think the employer can only do this if the side hustle is affecting the employees performance in the work place.