Earlier quoted context omitted.
Insider trading isn't necessarily bad for _the market_; it brings more information to the market faster, so prices more accurately reflect all factors. Compare for instance a hedge fund using satellite imagery to notice that a refinery has much less oil and trading on this information vs some exec at the oil refinery trading on that information. The former is completely accepted, because of the information it brings…
I think this view is very naive because it doesn't take into account the perverse incentives legalising insider trading could create. Insiders often also have power over the behaviour of a company, so for example a corporate leader might short his company's stock and then take actions to ruin the business. Or they might release plans for some action that would be seen negatively in the market, depress the stock price…
Legalising insider trading wouldn't legalise such behaviour per se, as such behaviour would still constitute a breach of the leader's fiduciary duties. Look for instance at how Musk was prosecuted for doing that kind of thing; such prosecution was unrelated to insider trading laws, and could still occur without them.