They are not. MFN for Iran, Iraq, Syria, and Libya is a moot point since nearly all trade is banned with them due to their involvement in state-sponsored terrorism. Syria and Burma both are denied low tariff benefits under the Generalized System of Preferences. Besides, policies against individual countries have evolved in response to historical developments and the needs of U.S. policy. No proponent of MFN renewal would say that the U.S. should treat every country exactly the same way, regardless of specific conditions inside the country, the type of government it has, or the type of threat it poses to the U.S. or its neighbors.
China is a special case.
When you think about it, there's no valid reason for the United States, the world's leader in freedom, offering the same trading terms to China that the U.S. offers to other nations that do honor their citizens' human rights, and that do respect the rule of law. There can be no such thing as normal trade with the world's largest country, a communist system engaging in proliferation of conventional, nuclear, biological and chemical weapons. A country of which our State Department can say, there was not a single dissident active in 1996. A country which is violating commitments it made in an international agreement to preserve Hong Kong's institutions and way of life virtually intact. A country whose economy is built on prison labor and People's Liberation Army joint ventures with U.S. companies.