To be honest, it's to keep bandwidth hogs like me under control. 3 gigs is on the low side though, and pricewise it's a bloody insult compared to the rest of the civilised world. Telkom's infrastructure supposedly can't handle it. Yet if you're willing to buy up extra bandwidth, then they can. Go figure.
Since Telkom have a monopoly over ADSL connectivity and ISPs just on-sell the accounts, even ISPs like MWEB are basically beholden to the rules that Telkom sets down - and ISPs who sell extra bandwidth are basically selling you a second account's worth of bandwidth and taking care of the admin. It's noo *too* expensive, but at about R200 for another 3 gigs of traffic, it still raises my hackles.
In short: Telkom are monopolistic sharks. Viva deregulation viva!