Answer:
Well, because it'll be very difficult to tell apart one object from another when one perceives there is an overlap between the two.
Step-by-step explanation:
Let's remeber what Helmholtz said about perception, that it some times results from the assumptions we make about the enviroment without even being truely aware of it.
In this case, the umpire will have a really hard time judging wether it was "foul ball" or a "home run", given that his emotional status may pull him toward one call or the other.
Thankfuly, there are video-recording repetitions in baseball now.