You can sort facets in your application, not sure why you'd want the search engine to sort them, doesn't seem like a feature that belongs to the "back-end" search engine.
Say a query returns a million results, it makes much more sense to sort them on the search server and return the top 10 than to transfer them all to the application server and sort them there. Another use case is a custom ranking function which boosts the score of newer documents.
If you want to sort by anything other than the number of hits in every facet, then there is information about the facets that only the search engine really knows. For instance, which facet holds the most relevant results?
