you could not be more wrong. They can and will take the cash from you givving you a reciept and sending you on your way penniless and with a very short time to to file a claim to it. Then you will have to prove where it came from thru an expensive unfair procedure. [foul language] Doubt it? Checkout "in REM" laws & proceedings
You seem to assume because that happens in SOME cases that it happens in ALL cases. It does not. I used to be a lawyer for the federal government before leaving for private practice and know that from first hand experience. The government must have reason to believe that the cash is connected with illegal activity or have some other basis to seize the cash (i.e. a lien in favor of the government). Merely carrying a large amount of cash is not, without more, sufficient basis to believe the cash is connected with illegal activity.
While this thread concerned domestic flights, if someone comes into the U.S. from abroad with a large amount of cash and fails to declare it as required, that may provide a basis to seize the cash too.