In stock markets, insider trading is a big no not because it ruins someone's gambling habit, but because the entire concept of the corporation requires a certain amount of trust of financiers in financees. That whole pooling of capital thing, to do stuff that has too high a capital requirement to start individually. When shares are publicly traded, that trust is impossible when holders have to assume that they will b…
No, they have a different reason. Consider the consequences of a sufficiently large prediction market bet on whether or not will be assassinated this year.
Consider also the consequences of insiders and decisionmakers having an immediate financial incentive to take the other side of a bet whose outcome they control.