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.)
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.
With prediction markets, the "purpose" is information discovery, and "insider trading" actually helps (=> via information from insiders).
Disclaimer: I'm somewhat playing devils advocate here, I personally think that prediction markets are for now mostly an ineffective zero-sum game (and legalized gambling with all the drawbacks that brings).
But you don't usually buy the stocks from the company itself, do you? Unless there is some shenanigans with buyouts going on...