Legality is something objective. Legitimacy is subjective. However, when something is illegal, it's likely to be illegitimate for a lot of people. Using inside information is stealing. If I work for a company and I'm the first to know that something bad is going to happen to it, it's not fair (nor legitimate, nor legal) that I use that information to profit personally. Basically, I'm stealing from other investors. If outsiders knew that, they would be disinvesting (since they knew to be in disadvantage) and that would hurt the company. So, using inside information is stealing from other investors and, ultimately, stealing from the company.