Actually, Little League International does not mandate facemasks for batters helmets in baseball or softball. If they mandated them for softball, they would likely have to require them in baseball as well, and that is another can of worms.
My DD has always been required to wear one. I would love to see what would happen if a girl walked up to the plate without one! I have seen umpires send girls back to the dugout for a chin strap.