Well the thing is, people bought KH2 because either
1.) They own KH1 and want to know what happens
2.) Like action/RPG's and heard this one was gonna be good
3.) Own a PS2 and want a good game.
Now, if FM+ comes out:
1.) People with KH2 who aren't major fans, or haven't looked into this, will think it's a money-scam to sell more copies, and well look bitterly at the game.
2.) Majority of people will think there's no difference between the two copies, and will wonder why they can't just pay $20 for KH2 instead of say, $70 for FM+.
3.) People in the US have never been introduced to FM before, so when they see this, they will just think they can play the same thing for $20, and might hold a grudge against SE.
The others, which are
4.) Hardcore fans who have looked into the game, and are interested in buying
will be the only customer population left to sell to. Which isn't very high.