Mountain Crest is a way to allow movement through the region at a very slow speed, to differentiate from the pass.
AFAIK, we couldn't use Impassable Mountain, as the link for the pass would act wierd...
I'll look at those regions, but probably won't change them until a discussion with the designers. I'm not sure of the actual terrain there, and game behavior (movement limits) may have overruled the graphics..