The problem comes in a more specific case than that.

We know Twitter has filed for an IPO. Will it be good? Will it be bad? We don't know any of their financials. But insiders know.

Now, granted, they'll file paperwork to show them before people can buy, but that's (partially) due to the same thought process as 'insider trading is bad.'

Now, imagine a situation a year after Twitter IPOs: An all-hands meeting, where management tells everyone that they won't be getting their annual raises because finances are bad. Employees can exploit this knowledge, but those not in that (private) meeting cannot.

Any information asymmetry _will_ be gamed.