"Why should Eskom allow...?" - Because government has told it to.
And because government - from the President through to Parliament - decided to make the procurement and it is a good investment for the development of the national electricity system.
Government allowing a state-owned monopoly to defy formally promulgated policy and fiduciary commitments, thus blocking at least R58.8 billion of investment in waiting, is not good for our economy. Think of it as 1 - 1 = 0.
The impact of procurement from the plants mentioned in this article, and even of the full REIPPPP commitment of 6 400 MW, on total Eskom costs remains fairly marginal, even if only assessing for the short term. 
The Department of Energy has published comprehensive accounts of who is benefiting from the new power  plants (may one assume that you don't count the DoE as handlangers of foreigners?) - see for example the IPPPP Overview March 2017 available via their website
But do share a wild story if you actually have one