True, but I think (might be wrong) fixed line ISP's work on a tighter margin. Also remember how it works:
I buy a 3GB package from ISP A.
ISP A buys their bandwidth from Telkom / IS / Verizon, etc.
I pay a set fee for a 3GB package, lets say R 220.00 / month.
ISP A pays their provider (Telkom / IS, etc) for their total bandwidth used per month, for their whole client base.
If I only use 2GB of my 3GB package, that in turn means ISP A will pay their provider only the for the 2GB that I used, and not for 3GB, although I have already paid for a full 3GB. This in fact means that ISP A has gained the fee for 1GB from me, and can use it as they see fit, not having to pay their provider for it.
Remember that Telkom changed the way they charge the ISPs a whilst back, moving to a pay per GB system.
TelkomInternet is relying on this system (or so they say) to allow them to do softcapping. So my 1GB I don't use with TelkomInternet will be used by someone using 1GB over their limit, so TelkomInternet is not loosing money, but are giving what was paid for, although it was paid be me and not the other guy (although I doubt that it is so cut and dry).
If that makes sense at all. So you allow your clients to carry their bandwidth over to the next month, i.e. give them a chance to use ALL they paid for, and I guess their pricing for the "uncapped" packages will go up as well.