Personally I feel this activity should be banned, both internationally AND around the entire world.
The first (and only) event I watched featured muscular men punching one another in the face for sport. I was outraged. The fans cheered for more. It was utterly and remorselessly appalling in both manner and presentation.
The participants are all there by their own admission, and are free to quit whenever they want to. The referee will also stop the fight when a fighter can no longer defend himself. While you may think it's outrageous, fighters are highly skilled, dedicated and passionate about what they do.