Where I am in Canada, I've had exclusivity required for an employment contract but only within the employer's niche and for pay well above industry norms.

The problem over here comes when the work is structured as a contractor/client arrangement.

If someone is set up as contractor, even with a registered company, they can be considered a de facto employee and have all the protections given to regular employees. Additionally the client, as the de facto employer can get in trouble for not making and remitting payroll deductions.

Even incorporating won't save you. There are several criteria but, if you're the sole employee of a corporation, you're considered a personal services business. It makes you ineligible for any corporate tax reductions, an additional 5% tax, and virtually no deductions are allowed outside of payroll expenses; even supplies and directors' action payments.