Us "hard-over" 100 percenters and "I want one just like me" conservative candidate seekers have a hard time with anything that doesn't put this exceptional country ahead of all others, in all things. You know how it is with us "true believers!" We git all carried away with our obsession with America and want to protect it because we believe it needs special standards of care!!! (they're not makin anymore Americas with freedom, these days)
MFN status really only means that you won’t impose special sanctions against a country that you don’t extend to everyone else in the absence of a special negotiated trade agreement.
A rule that says China gets hit with a 20% tariff on all its exports to us would be contrary to MFN status, unless we hit all countries with a similar tariff.
Theoretically, we could hit China with a 20% tariff and still have them in MFN status, if we did it with everyone else.
Free Trade agreements are different in that they require the nations involved to lower or eliminate tariffs between the two countries. Those supersede MFN status, and generally lower prices for consumers in both countries by removing, or at least reducing, the government taxes on imports and exports.