It's illegal for them to deny entry to U.S. citizens.

I'm the author of OP's post. Can you provide me more information about this? I've always wondered if that was the case, but I was once given a lengthy interview in the Amsterdam airport on my way to DEFCON last year, which gave the opposite impression.

If you are a US citizen, US immigration cannot legally refuse you entry. They can make your life miserable (via private questioning, etc.), and US CBP can also make your life miserable (going through all your stuff, questioning you at length, etc.), but they are legally required to allow you into the US if you are a citizen. If they can pin something illegal on you, they can of course detain you and refer you to local or federal authorities, but they must (as a part of that) allow you back into the country. They can't force you to stay in the international zone at the airport, and can't send you to another country.

Now, this only applies to US citizens returning to the US. A US citizen arriving, for example, at Amsterdam, can certainly be turned away for any reason whatsoever.

Thanks for this information. In my example, I was departing Amsterdam for the U.S.