Waiting period imposed unfairly

I recently added my wife on my medical aid. She has been on a discovery medical aid since May 2016 without any interruptions and no chronic issues or any complications. However they imposed a 3 month waiting period on her. After challenging it, they still maintained that the underwriting is correct and the waiting period remains. Honestly looking at her situation, it does not make sense. What is the basis of the decision to impose this 3 month waiting period? Unless Bonitas just wants to be difficult. Please help me understand.