If we compare with waterbending, the bender can change water through its physical forms, solid, liquid and gaseous. I believe it should be a natural skill for all earthbenders. So...Why doesn't the same apply when the subject is turn earth into its liquid form?
Of course, at least in theory, lavabending would require a larger amount of energy than simply turn ice into liquid water, but perhaps, with proper training, they all might be able to do it.