well, if on the field during batting practice or other practice, they need to have someone paying attention.
If I recall correctly, a minor league 1st base coach was killed by a foul ball. That’s why base coaches wear batting helmets nowadays.
And, teams have been extending netting along the baselines, to protect fans from line drive foul balls. There have been fan injuries due to foul balls.
This happened back in 2007.
Coolbaugh, 35, dies after being struck by ball
http://www.espn.com/minorlbb/news/story?id=2945798