You see, words mean things, apart from what you believe.
“Free trade” is used all the time by politicians, so I assume they use the term to deceive people.
Free trade is one of those things, that I would content exists only be degree. Like any theory, even if the theory is correct, the benefits will decrease the more you deviate from the perfect implementation of the theory.
Free trade within the 50 states is good, but mainly because the gap between states is not as big as the gap between the US and the countries that employ slaves, such as Malaysia. There is some wiggle room between the states; e.g. unions or no unions, so there is room for some experimentation.