Thank fortune, one may be bald without being piebald.
The element pie, in this case, relates to the magpie, a bird with feathers splotched with two colors, a bird called pica by the Romans, whence the English pie.
Originally one of the two colors was, as with the bird, always a white spot, having thus the appearance of a bald spot.
Actually, therefore, piebald originally related to the white-spotted pie. (See also skewbald.)