There is a line that needs to be drawn somewhere in terms of what can and cannot be allowed in contracts, the question is where to draw it. The other obvious point is whether both parties are acting in good faith, which is why there are laws about fraud on the books (even if they aren't always enforced for some financial industries).
You had just said it's not appropriate for governments to insert itself between two informed, consenting parties, because it is paternalistic and not part of a free society.
Also what does market efficiency have to do with anything? There are always going to be asymmetrical distributions of power because information is asymmetrical.
A lot of what the SEC and FTC do is require appropriate information disclosure to protect consumers from deception. The SEC requires financial disclosures, auditing, etc. In the FTC's own words: "Federal Trade Commission has a broad mandate to protect consumers from fraud and deception in the marketplace". The idea is to make sure people have the information that they need to make informed decisions. Not to tell them what decisions they are and are not allowed to make once they have that information.
And to the contrary, both the SEC and the FTC dictate what types of transactions take place in respective domains - unless you're arguing that the prevention of MLMs and ponzi schemes infringes on your freedoms.
As for who's going to enforce my $100k agreement, the same people that enforce any other contract, the courts.