Yes it's legal, and I'm pretty sure most of the countries in Africa that allow it are doing it as a wildlife-saving initiative. In an attempt to preserve what little wildlife they have left, and in the face of apathetic authorities, they are making the animals "valuable". Not "valuable" in the sense you and I might use the term... but valuable in the sense that the authorities have to sit up and take notice of the conservationists because now there is a lucrative market, ie. hunters paying through the nose to come and kill a leopard. They are trying to change the so-what attitude towards poachers by sacrificing some of the animals to rich sportsmen; the poachers are now violating an economic resource.
