Pop quiz. What happens when you bring an element of harmony to a altered world?
Sunset said this in the movie but we never got an answer to it. But I think that the when you bring a magical element to a world that doesn't have magic it becomes unstable and unpredictable, letting anyone use it in ways that shouldn't be used or in ways that shouldn't work.
But hey its just a theory, a brony theory.

Keyword: "presumably".