Mirriam has been working for Solar Limited for the past 10 years. After completing her Master's degree in Business Management, she was promoted to the position of senior manager. In terms of Mirriam's appointment as senior manager, she signs an addendum to her contract of employment prohibiting her from joining any trade union. Even thought she very much enjoys her position as senior manager, she quickly comes to realise that even though she has taken on a senior position in the organisation, the employer remains in a much stronger bargaining position.

After 3 years in the position of senior manager and despite receiving many accolades for her exceptional work standard, Mirriam has not received a salary increase. Mirriam approaches the human resources manager to enquire why she has not received and annual raise as her subordinates have. The human resources manager tells Mirriam the increases are negotiated by the respective trade unions and that instead of complaining she should be happy to still be employed.

Discuss the constitutionality of the approach followed by Solar Limited towards their senior managers and if there is any legal protection for Mirriam in the given circumstances? In your answer refer to relevant case law and legislation.