I didn't see the show and no way would I want to shoot a rhino with a dart from 15 yards but............... here is your enlightenment.
The whole reason there are huntable populations (darts or bullets) of rhinos is a direct result of the selling of tags to hunt them in the first place. At one time the white rhino (I thnk) was left to only a couple breeding pairs left in the world.
I can't remember all of the details but a man who aparently had these rhinos on his land, or somehow aquired them, or what ever, sold off a tag for an ungodly ammount of money and began funding research, and conservation efforts for these rhinos If it were not for the incredibly rich "dumb arses" that like this sort of thing there would not be any rhino left on this planet to see, much less look at in National Geographic.
Basically he made it so the rhino was worth more to the governments alive then they were worth dead which prompted the governments involvement to initiate protection for the rhinos and begin capturing, prosecuting, or shooting on sight poachers of rhinos.
Aside from that once the rhino is drugged and down valuable data is collected from the animal for scientific research which has a direct impact on the overall health and population of these animals. It's not just for a photo op with a rhino.
It is tragic that this particular rhino had to be killed the way it was but get and know the facts surrounding a hunt like this before you start bashing people like this.