Anyone that doesn't think it was fair should think about a bunt that goes down the line foul and sneaks back into fair territory. As other posters said it can cross back and forth a dozen times before the bags at first and third, it's fair or foul based on where it is touched first or where the umpire rules it stops dead. Only exception is if it hits something that is not part of the field of play in foul territory. At that point it's automatically a dead ball and foul.
In a side note Mangum should have run. He is a hustle guy and hustle guys always run it out. I am too young to have watched all of Pete Rose's career, but I remember the end. He would of run, unless he knew for sure he could not make it close. At that point, he would have turned around and socked the catcher in the stomach before he could field the ball instigating a bench clearing brawl. Hustle guys should never be an easy out. Learning experience for Jake.