I've been taught:
A) it is a balk, but don't ever call it!
B) try to prevent it from happening. When you see the catcher setting up outside the catcher's box (if you even have one), tell him to "stay put until the pitcher tosses you the ball".
I agree with the comment that some coach read a rule, took a course or had something happen to him once upon a time, to be complaining about it.
