Is it? We've had unrestricted trade among the United States for over 200 years.
Would we as a country be better off, or worse off, if we had tariffs, quotas, and trade wars among the states?
Midwestern states wouldn't be as efficient (cost effective) as Florida at producing oranges. But, hey, put a high enough tariff on Florida oranges, and you've got orange production in Wisconsin! (Would that be good or bad?)
One very simple example, but I hope you get the idea: the USA is the most prosperous country in history because of free trade among our states.
The benefits of free trade are no myth. But implicit in free trade is "fair trade" -- a level playing field. Otherwise, the gains from specialization (Florida more cost efficient at orange production than Wisconsin) all go out the window.
Thanks for making my point. What the so called free traders like Levin espouse is neither free nor fair. Your comment about free trade within the USA is a non sequitur.