The fundamental reason why South Africa remains out of line with the rest of the world on broadband billing (apart from Telkom, which is obvious) is the fact that a large part of the market is dominated by so-called "mobile broadband".
As has been done by mobile networks almost everywhere, the mobile operators have protected their lack of backhaul and their price cartel by making download size (caps and associated "data bundles) the de facto benchmark for broadband billing in South Africa. As a result, most consumers really don't care about things like speed or performance, and measure everything in Rand/MB (or cents/MB if you have the right provider or package).
This plan by a cable company is out of line with the billing conventions in the US fixed line broadband market. The model would probably not raise an eyebrow on a US mobile network (or any other mobile network around the world). In South Africa, most consumers would view it as perfectly normal. If (most) South African broadband users were prepared to pay for speed and performance, rather than simply measuring the price per Megabyte, providers (OK, except for Telkom) would price differently.