The only thing I can think of is that some visualize the front of the plate as a continuation of the foul lines. Or when a batted ball hits the plate and then the batter, someone started the myth that it was foul because of the plate. Maybe they even knew that a fair ball hitting the batter (in fair ground) is not foul. It's like when a batter is out for being out of the box, everyone always tells him/her that he/she stepped on the plate.
Best of all, it's why they need us.
