Except that in elbows, there's galvanized pipe exposed past the threads. Really not much of an issue, but galvanized pieces are always more $. That alone should be reason enough.
I wasn't trying to say it was proper workmanship or not just that it wouldn't cause harm. As for the cost issue it depends on other things. Contractor could be using a butt load of galvanized and black is only used for gas.
