How? There is no law requiring travelers to report cash movements if they stay within the United States. I could legally carry a suitcase with a million dollars cash in it, flying from Peoria to Pocatello. It's only when the individual traveler is incoming or outgoing internationally that the $10K reporting threshold kicks in...this is explicitly stated.

Now, whether my million dollar suitcase would invite theft, or whether various law enforcement agencies or TSA clerks might be suspicious, are different issues. Is it any less legal if I didn't fly? What if I purchased a couple of mattresses, stuffed my million bucks in there, loaded the mattresses on the U-Haul and drove from Peoria to Pocatello. Same effect. Cash was in Peoria. Now cash in Pocatello. Who do I report the movement to?

Repeat once again: Cash is not equal to Crime. At least not yet, in the USA.