What if the person isn't mentally ill, but doesn't reliably have enough to eat to survive adequately, and can get it by fighting for the entertainment of others (who do have plenty to eat, and more to spare).
Is that ethical, because it's a rational cost-benefit analysis?
There are few people in the US who don't have enough to eat to fuel their bodies, but in other countries there are more. I am not trying to describe the particular people in "bumfights", who I don't know, but hypothetically working the boundaries of this analysis.
I also don't in fact think that people can be divided, in binary fashion, into those who are "rational" and "not rational".
It seems pretty clear to me that enticing desperate people to fight each other, for the entertainment of others who find their desperation emotionally engaging, and for the profit of the promoters -- is unkind and disrespectful and an attack on their dignity. Even if they are not "mentally ill." It seems hard to argue otherwise. Is it how you would treat a relative or friend? To hypothetically say, well, sure it's unkind and disrespectful and an assault on their dignity, and I wouldn't do it to someone I loved, but it's not unethical to do to make a buck or a laugh -- what is the point of "ethics", then, exactly?