Employers may enforce restraints of trade by placing certain limitations on the freedom of employment and thus rendering employees ineligible from trading within the same industry after they resign.  Such restraints are additional clauses added to unique employment agreements.  As this is an addition to a contract and a restraint of trade is considered to be above and beyond an employee’s call of duty.  An employee is required to agree to that which is stated in the employment contract in order to accept the propositioned role.  Although employees might not agree they should be aware of the restrictions before accepting a role.

Source: http://www.thesait.org.za/news/109152/Restraint-of-Trade-Explained.htm