Put a stop to what? Torturing, raping, murdering, and scalping hosts? All of that stuff is by all appearances completely within the norms of what the guests usually do in the themepark. It is telling that not a single depiction of the guests in the pilot episode was positive. Even the staff seems to have a detached disdain for the guests.
The man in black? The staff probably see him as one of their best customers. He probably gets the red carpet treatment from the staff and is at the top of their promotional mailing list. They know exactly what he is doing and condone his every action. Hell, they probably design characters and storylines to suit the tastes of him and other reliable customers like him.
The man in black isn't an aberration, he is presented as the very embodiment of the "guests" as a whole, much like how Delores is presented as the ideal host. The fact that they both have deeper layers to them shows how the entire world has deeper layers to it.