why they don't want people on the field

One moment that absolutely made me chuckle this past week was the sight of a fan running onto Houston's Minute Maid Ballpark, and escaping the authorities by jumping over fences and scaling parts of the ballpark until he was out of the ballpark.  The link to watch the escape is here: http://youtu.be/X7B4gic1-qU

While this might seem amusing, there have been a few instances where the athletes and other officials on the field have been victims to fan attacks during a sporting event.  Tennis player Monica Seles was stabbed in the back on the court in 1993.  In 2002, Kansas City Royals coach Tom Gamboa was attacked on the field by a father and son.

Tom Gamboa being attacked in 2002
photo from ESPN

Since officials don't know the intent of fans, drastic actions, such as the tasing of a fan in Philadelphia might become more common.

a Philadelphia fan getting tasered
photo by Matt Slocum