That's fair. Insofar as the purpose of the stock market is to allocate capital to companies the market predicts are most likely to succeed, you could argue that trading stocks based on insider information should be legal (though not insider trading by those with the ability to influence outcomes).

But I think the way our current system solves this is by forcing companies to disclose important insider information to shareholders all at once, rather than by letting insiders profit by leaking the information through their market transactions. (The one possibly being related to the other, because insiders who could legally profit by keeping information secret rather than disclose it would be incentivised to do so.) I don't think a similar restriction is feasible for prediction markets, because often the thing you're trying to predict is not something that can be legally cajoled into giving up its secrets.