Whether for dagger handles in Yemen or purposes of medicinal quackery in East Asia, rhinos have been hunted heavily for their horns for centuries. The three Asian species have the added threat of habitat loss, but it truthfully doesn't seem to matter in the end - the case of the two African species shows that poachers will go to the ends of the earth to secure a rhino horn. When I was in South Africa recently, park rangers told me that they never, ever even alluded to a rhino's whereabouts over their radios, for fear that eavesdropping poachers would learn where to find their favorite prey.
I've heard the idea floated in the past that maybe we should accept the inevitable - that the demand for rhino horn is too huge to fight - and try other strategies than protection. Dehorning rhinos in the wild has been tried... but it reduced the ability of females to protect their calves, and poachers began shooting hornless animals out of spite. Besides, the horn grows back, and poachers find even the smallest nub to be profitable. Some folks have mentioned farming rhinos for their horns - lopping of horns as soon as they are large enough to sell, then waiting a few years for a new horn to grow. These suggestions are usually made by people who have no idea how demanding a task breeding rhinos can be. Also, you still have to worry about poachers raiding your breeding stations to take the horns for themselves.
South Africa - which has invested a lot of money and man power into protecting its rhinos - has accumulated quite a stockpile of confiscated horns over the years. They want to start selling it off, hoping to ease the demand for rhino horn and possibly make poaching unnecessary. Others fear that this will only make it easier for poachers to sell their illegal horns. Besides, those confiscated stores will run out at some point. Do we really want to be adding to the demand for rhino horn?
Read the comment section of any article of rhino poaching and someone will inevitably propose a more draconion solution. Poison rhino horns and let them slip into the market. Let people learn that you're playing Russian roulette every time you use "horn" medication. If I thought that this would only affect the people you actually were responsible for rhino poaching, I might be on board with it. But what happens when someone gives rhino horn to their small child with a fever? Besides, people will only be willing to pay even more for "pure", uncontaminated horn.
The only way to protect rhinos, in the end, is to eradicate the demand for their horns. The Chinese and Vietnamese governments need to step up to the challenge of not only actually enforcing laws against the use of endangered species in Traditional Chinese Medicine, it needs to educate people that these "medicines" DO... NOT... WORK! Rhino horn = fingernail! You are paying a fortune to drive a magnificent animal to extinction for absolutely no value to yourself, beyond the placebo effect! Only when we are sure that no one wants a rhino horn can we be sure that no one will kill for one.