Why is it illegal to invest in something you have inside information on?

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For example, you're friends with Bill Gates, and he tells you Microsoft is selling a new product, but the information hasn't been made public yet. Why would it be illegal to buy stock now if A.) It wouldn't have been illegal if you hadn't heard about it, and B.) It wont be illegal when it is made public? I just don't understand logically where this law would come from. It would seem that the only difference is having an edge against the public, so what about that is illegal?

Why is it illegal to invest in something you have inside information on?