For Ant - as Perdition noted, their pricing and service structure is geared towards ensuring the maintenance of their client base, so you're not wrong in your thinking.
Most businesses adhere to pricing formulae which promote the realisation of their specific business objectives, whether this is maintenance or growth of the client base, maximising of revenue levels, expansionary growth into new markets, and so on. I think the term used is 'segmented pricing' - this can even mean deciding on 'loss-leader' pricing in certain segments, depending on the business objective concerned (retailers usually do this quite often, as they are more concerned with pushing sales volumes, due to their profit being derived from discounted margins for bulk volume purchases from their suppliers).
In the case of Telkom, they're endeavouring to retain their lucrative corporate clientele, so the obvious solution would be to maximise the pricing for the ADSL service to that which the market will bear (without material loss of new ADSL clients), and couple such pricing to a crippled delivery offering which would cause the corporate client base to lose interest. It's a strategy which works in the short term, but which is destined ultimately to fail. They now have some competition in the form of Sentech and the imminent SNO, they've been zapped by regulatory decision (the Competition Commission), and they've pee'd-off their client base. Quite frankly, their management should be fired!