that the rangers will turn around and start poaching rhinos themselves

None of this is difficult to understand - it's no different than the right-wing death squads the US trained to fight their little "War On Drugs" suddenly and not-so-mysteriously ending up the biggest players in the drug-smuggling business.

Who did you think these "rangers" work for? They are pigs - that's what they are. And like all pigs, profiting from the illegal things they are (supposedly) "preventing" is merely one of the unspoken but universal perks of the job. As long as their violence serves the capitalists that wants to exclusively loot and pillage natural resources, everybody in power will turn a blind eye. And it's not just the trade in animal parts - don't be surprised when it's discovered that these privatized goon squads being lauded in the media for their (supposed) "anti-poaching" activities take their orders from multi-national mining corporations or companies that want to exploit local populations as cheap labor.