Simple really, again it's all about the money and marketing. If people have a affordable choice other than DSTV and SABC, who will end up watching the shait they broadcast on SABC? In turn who will want to advertise on a TV network with verry few viewers? In turn that means less money to line the fat cats pockets, which means they have to get off their backsides and start to work, providing competitve and ENTERTAINING content to the people... but that could proove too much for poor old SABC.
IMO you've hit the nail on its head there.
TV adverts are a big source of revenue for the SABC, MonoChoice and e-TV, and advertisers factor in demographics and cost when deciding which channels to advertise on and how frequently to have an advert broadcast.
It is reasonable to conclude that the SABC is scared that advertisers will spend more money advertising on the Pay-TV channels than on SABC, if the demographics say that's the best way of getting any particular advert out to the desired target audience [subset of consumers].
So forcing the Pay-TV network operators to carry SABC channels, implies that advertisers might still consider advertising on SABC channels as Pay-TV subscribers will have easy access to adverts on SABC channels broadcast via the Pay-TV networks.
The flaw in the SABC's logic, is that there has to be something worthwhile on the SABC's channels for Pay-TV subscribers to even consider watching the SABC channels and adverts.
About the "must-pay" [to the SABC] aspect, I don't know how the SABC1&2&3 broadcast content currently gets transmitted from the SABC to DSTV, but I expect it involves one or more expensive fibre links - probably that MonoChoice currently pays for [the link(s) that is] - unless the SABC uplinks its broadcasts directly via satellite for re-broadcasting via DSTV. If expensive links are required to get the broadcast content from the SABC to the Pay-TV network operators, then the Pay-TV network operators would already be paying for the initial terrestrial transmission of the content, and then the SABC still wants the Pay-TV network operators to pay for the actual content itself - that's before taking into account the actual satellite transmission broadcast costs for actually being forced to carry the additional SABC channel content...
Repeating what others have already posted: the costs associated with being forced to carry SABC channels content, would be passed on to Pay-TV subscribers, who probably don't want the SABC channels content anyway - I know I can live without SABC channels on DSTV etc if it means paying less.