Honestly, the practical outcome of a ban is that officials and their families are forced to dump stocks at random moments, often taking big losses or moving money into blind trusts they don't control. But if owning stock creates a conflict, then owning a house in a district you legislate for is a conflict too, and so is owning a car from a company you regulate. If that were true, public officials would have to live in a bare room with nothing but index funds. The real problem is insider trading, and that's already illegal.