Being as Nairobi park is very close to the outskirts of Nairobi, the capital of Kenya, I suspect to unromantic & more prosaic truth might be that the armed ranger is not so much for protection against charging or dangerous animals or poachers but rather common or garden crims/thugs call them what you like. The Park does have big cats as well as Black Rhino & Buffalo though so I'd be happy to have an armed guide along for whatever reason.
Steve