That's just the way Vodacom does business. They have even more contracts.
My opinion:
Confuse the customer and somewhere he/she will miss a higher amount which will make him/her spend more. Furthermore, by introducing new tariffs to which users can switch to (which might actually truly be cheaper) many users are kept on the existing one (perhaps they don't realise that there's something cheaper available, or that they need to switch) which are more expensive (and thus bring VC more profit).
If they just went and changed everybody over, then everybody would get the benefit automatically and the network would lose a lot.