The reason why it is important to Disney to animate nature is so they can express and portray the realism of it, and keep memories of that contained in films in their image for years to come. One example of this would be when the animators had brought in deer fawns to really grasp the details of the characters in Bambi. Disney usually takes liberties away with nature in a way that what's happening is truthful, but not real. Going back to Bambi, for many people, it is a film that portrays death to the audience by man, which is true in real life for a huntsman, yet is not real, as it is only an animated cartoon of it.
One way that the nature films represent American culture is by the characters taking on different walks of life, such as race and traditions. In the Lion King for example, you had different characters with their own voices and personalities, such as Timon with his new york like accent and laid-back attitude, compared to scar, who had a British upper class about himself and was about cunning and finesse, with strict discipline. Yet due to their stereo typical ways for some races, they fail to represent the American culture. In fantasia, with the nature scenes of minotaurs for example, fishes have feminine like features, and inanimate objects have a sense of some heritage to them; last time I checked, for the mushrooms, that is not how all Asians portray themselves to be, what with the style of movement and the mushroom head/Chinese hat deal.
I believe the animated nature films created at the time of war plays into it in such a way that it reflects upon the roles of the characters within the film, similar to real life. Again in Bambi, the mother of Thumper asked him a question along the lines of “what did you father tell you,” which could imply that he's already gone in war, yet his authoritative lessons still remain a vital part of the family towards his children; the deer also seem to have a marching unit like feel to them when The great prince stag makes his presence.
One reason as to why Disney may make stories as his own that originate in non-American cultures is because they wish to show existence of different cultures besides the American culture already. Take for the red men for example in peter pan; sure, they are of a stereotypical nature in which Disney depicts them, but at least anyone who watches it will either know of indians, or would ask more about them. They also show us the way of the chinese existence in Mulan as well.
