As this article by Gary Galles states:
"Why do politicians endorse free trade, yet devise 'but this, that, and the other' excuses for protectionism? They talk of commitment to free trade, but create restrictions instead because most people's commitment to narrow self-interest exceeds their commitment to principle. There is always something, whether it is environmental concerns, the trade deficit, unemployment in particular industries, self-sufficiency, national defense, or a desire to pressure other countries to change their policies, which can provide political cover for self-serving actions."
"This 'I'm for free trade, but' approach ignores America's history. Much of our early economic success was because the Constitution abolished states' attempts to take advantage of each other through restrictions on interstate commerce, creating the world's largest free trade zone. Everyone benefited, as no government could impose extra burdens on mutually beneficial trades just because shipments crossed a border."