But why not? Isn't the word "fantasy" in Fantasyland?
I feel like all the princesses fit into the over arching theme of "Fantasy" and are therefore allowed to be seen in Fantasyland. Doesn't mean they all are at the same time, but that's because other lands such as Adventureland and Frontierland fit their respective stories better thematically. Same reason why in Epcot you see Belle in France, or Snow White in Germany, or Mulan in China. But they aren't in those areas because they are forced to remain there due to the fact that their stories don't fit the theme of fantasy.
Obviously we will have to wait and see, but from the looks of things PFH seems to be a place where all the princesses come together, regardless of background or cultural identity. What matters is the "princess" theme, which despite cultural differences, Mulan, Tiana, Belle, etc all fit together. In the concept art you can see Mulan and Tiana on the walls. And in the BBB, little girls can dress up as any princess. You don't have to go to a certain area in the park to dress up as Jasmine and such. It's all done in Fantasyland in the castle, because princess and fantasy go hand and hand. Yes princesses fit better in their respective backgrounds, but if areas are built where the theme is princesses in a land that's dedicated to fantasy, I don't see an issue.
Also you said Story Book is a ride, so any type of fantasy goes. Well technically, PFH is an attraction in Fantasyland based around princesses, so why can't any type of fantasy princess fit there?