Free markets do not necessarily mean that everyone has equal access to information. We've got market regulations so that less sophisticated retail investors have a chance against the more sophisticated / secretive groups with more money / knowledge / power than us.
A laissez-faire market would encourage secrets as a means to protect ones own capital. To force some people to give up secrets (ie: regulations against insider trading) is itself, a regulation, that already moves us away from truly free markets.
Which is a good thing. I think a laissez-faire market is naive. There's benefits to free markets, but also downsides. We as a society should try to build systems where we get the benefits of free markets, but place regulations on the behaviors that cause societal harm.
These rules and regulations we as a society follow (and ideally enforce) have problems. But the way to fix things is to improve the rules so that they're fairer and more comprehensive. Maybe some rules are too hard to be enforced and should be tossed away (even if in a hypothetical perfect world, they'd be net-beneficial... if its too hard to implement or enforce, then there's no point in practice).
"Insider Trading among Congressmen" is more of the latter IMO. We all would like it if Congressmen played by the same rules we did. But in practice: how would we even write a law that hampers this behavior from Congressmen? Who will keep track of which stocks Congressmen are trading, and which committees they are a part of?
If we can't enforce the rule, then the rule doesn't exist (even if we write down the law into our code or even the Constitution, without enforcement, the law is meaningless)