Killing young, rogue (unsocialized) male elephants probably saved the life of rhinos, a truly endangered species. Young male elephants are notorious for killing rhinos for fun.
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And I have to ask again, what is the reason for picking rhinos over elephants to survive? It's occurring in nature, not man made, and how many hundreds of years have rhinos survived being 'picked on' by elephants?
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A quick web check revealed, not surprisingly, that while rhinos are on the critically endangered list, African elephants are not. From a wildlife management financial standpoint, the revenue gained from the greater goes to support the lesser. Where have we heard that before. 
We do pretty much the same thing. US wildlife management geared towards protecting endangered species gains financial support by the sales tax on ammunition, part of which is derived from the tax on ammunition used for hunting non-endangered species. So we're swapping deer for bald eagles.
Pittman–Robertson Federal Aid in Wildlife Restoration Act
http://en.wikipedia.org/wiki/P...life_Restoration_Act
My guess is, and it is only a guess because I can't / don't speak for African nations, that this elephant for rhino is their way of obtaining financial resources.
There was a special on one of the nature channels where it showed the resources used to protect the black rhino. Their movements were under surveillance in the air and ground by armed escorts like a convoy in Afghanistan.
The Asian elephant, pictured above, is endangered and I can only presume it likewise is protected.