Insider trading is a balance.
On the one hand, we want market prices to be accurate. This means we want people with material information to trade on that information.
On the other hand, we need some fairness in a market. This is mostly to ensure people keep trading. In a world were inside-info is commonplace, trading without it is just stupid. This would cut off a lot of people from investing.
The line needs to be drawn somewhere. The US approach of insider trading requires a broken 'duty to keep secret' isn't nice, but considering the above trade-off I think it is better than "All non-public information is off-limits". Especially because it captures the 'most disruptive' form of insider trading: people who work at a company that is getting acquired / going bankrupt.