The comments mention that the rules were changed to disallow out-of-band bargaining after this event, which is important because this is one of the core solutions to the Prisoner's Dilemma (which is what they're playing, and is not a zero-sum game). The solution in general is contract law.

The verbal agreement "If you do this and I do that then I will split the money 50/50 with you after the show" is a verbal contract which (due to the cameras recording the event) is nevertheless indisputable. You are giving them a task to accomplish in exchange for a promised financial reward. They have all rights to sue you in court if they did what you asked and you didn't pay them for it.

What's really interesting is that he promises to split 50/50. In theory a rational actor who could commit to these things absolutely would say, "Look, I am pre-committing to steal, and I will give you $100 after the game if you choose split." Then it is still rational for the other guy to choose "split" (getting $100) rather than "steal" (getting 0). However most people, in study after study, will be insulted by the disparity of only getting 1% of the winning, and the insult is magnified by the camera: it is public shame. So in some sense being insulted by unfairness introduces out-of-band costs that also tend to pull you to splitting the money, and splitting it more fairly.