Well, first of all, the batters do usually know it's coming. Second of all, it's
not at their grill, and third of all it's not intended to injure. It's intended to send a message. You'd be hard pressed to find intentional bean balls that have injured a player. Very rare. Batters are more likely to be injured from a wild unintentional pitch.
9/10 times a player knows when he's going to be hit intentionally.