I cant think of a reason why MLB wont add protection for the fans sitting close to the field. A child was hurt last month and now another this month. Each year this seems to happen several times. Last year a woman died at Dodgers Stadium after being hit.
Even for those fans watching the game closely you cant expect them to be able to react in time, everytime, a ball at 100 mph heads into the stands.
What is the hold up?