I think it's that Buchanan and the rest of those who style themselves "palaeoconservatives" are basically embracing an idealized form of national socialism.
They strenuously support unions and protectionism and others forms of socialism while embracing nationalism instead of internationalism like other socialists do. This nationalism seems to be vaguely based on race, but they refuse to be pinned down on this (exceptions to this vagueness are Jared Taylor and the late Sam Francis).
Strenuous supporters of protectionism in American history include George Washington, Alexander Hamilton, Abraham Lincoln, Teddy Roosevelt, Calvin Coolidge, and Robert A. Taft.
Strenuous supporters of free trade include Woodrow Wilson, FDR, and Bill Clinton.
Attacking protectionism as "socialism" betrays a monumental ignorance of American history, if not an actual hatred of America.