My understanding of it: America got a lot of extra features in the regular releases of KH1 and KH2, and the Japanese fans complained a lot. Square Enix got scared, so they made rereleases of the games and named them the "Final Mixes" because they were not going to release any more extra features for those particular games, I guess. NA's KH1 had Phantom, Kurt Zisa, Sephiroth, and Ice Titan as extra bosses that the JN version did not have. NA's KH2 had some extra stuff too.
Basically, the Final Mixes are not exported because the NA versions of the games have more content than the JN versions. To remedy this, they could simply NOT GIVE THE NA VERSIONS EXTRA CONTENT. But instead of doing that, they decided to continue to be unfair...