[joebob42]

I guess this can sort of flow two ways, though. A politician might know a law is coming up, and so buy a stock that is going to benefit from it: your argument is convincing there. But they might also hold a stock, and because of that they might feel more strongly one way or the other about a particular law: that feels like a problem to me.

Unfortunately there is bleed through between these, and no good way to distinguish the two except ok a case by case basis, which seems to me like a credible argument for forbidding all trading.