The thing is you're still thinking of these insiders as someone who just got a juicy stock tip from a relative.

The much more serious problem is when these insiders actually have their hands on the levers which decide the outcome. It's really no different than a mobster who bets a bunch on money on an unlikely outcome then threatens one side to throw the match.

What possible economic benefit is there to society to allow ordinary people to bet in markets like that?

Would you really like to live in a world where "Will we nuke Iran?" Is a bet you can make? Then someone in government sees how much money they could make if they bet yes & push the button?