Because silo shots (straight up) typically start to arc back towards the plate, or, if otherwise uncaught, carry enough spin such that they take bounces to the foul lines. I actually commend the PU on “reading the field” – skinned, probably hard-packed, grassless infield and an artificial/portable mound are ingredients to bad hops. If the ball had not been touched, and bounced-or-rolled foul, would it not have been a foul ball?
So why not tack on “... if fair!” onto the verbal declaration?