Dual sim disabled

Please can someone tell me as to why Vodacom locks the second sim on dual sim phones. I currently have a company phone and personal phone and was considering obtaining a dual sim phone to avoid having to carry two phones.When contacting Vodacom I was informed that the ons sim is locked. Upon enquiring as to what the reason was the vodacom employee had no answer and said that various clients have raised this issue. When asking what alternatives I have the staff member informed me that the dual sim is not blocked for Cell C and Telkom so are we now forced to explore moving to the competitor.