alt Hacker NewsIt's in the name: Prediction market. The point is to predict an outcome, insiders will naturally be better at that than non-insiders.
Though I think where things start to get a bit more insidious is when the "insiders" have access not merely to inside information, but the ability to change the outcome. That type of insider trading should be banned IMO because it works against the purpose of prediction markets as a tool. (Though the extent to which banning that is possible is debatable.)
That isn't very convincing, as the stock market itself is largely a prediction market. People buy stock to bet on future success, whether that manifest in the form of stock price increases, splits, and/or dividends. It's merely a much more narrowly-focused prediction market.
For that very reason, insider knowledge, and especially the ability to influence future outcomes, become the subject of heavy regulation. And, the lack of such regulation for congressional members is also why their net worth tends to skyrocket once entering office.