Some people have persistent decisionmaking flaws that make them engage in compulsive, self-destructive behavior. Sometimes these bad decisions cascade in a self-reinforcing cycle -- as with intoxication and intoxicant addictions.

Are these people fair game for however much profit can be extracted as they self-destruct? Should competing firms race to break them first?

You are wrongly assuming you are immune to this kind of lapse. A friend of mine was recently admitted to a psychiatric institution, while there has been no previous indication whatsoever that she wasn't completely sane. Being addicted is being halfway there: you have lost the capacity to think in a sane, self-preserving, way. Everyone faces that risk and I would damn well like if people helped me when that happened.

Someone like Watanabe should be helped, instead of abusing his shortcomings. Everyone that allowed him to continue gambling is morally responsible. What would you have done if it was a friend? Your father? That is the criterion you should use to judge these cases.