The bottom-line is this. Every fan of a reasonable age knows that running out onto a pro sports field will get you a) hurt, b) arrested and likely c) both. You have had more than enough incidents through the years (read here and here for a couple of examples) that prove that a fan on the field can be a bad thing. Craig Calcaterra over at Circling the Bases was spouting an absolutely ridiculous argument that tasers are somehow dangerous. Sure, you shouldn't be playing around with one but a) they are effective at subduing offenders and b) every police officer in America is tased during training so they know what it feels like. If they were that dangerous, they wouldn't be used as much as they are. When that kid jumped down from his seat and on to the field he became subject to whatever means was necessary to bring him into custody. Watch the video, the kid runs around and is clearly quicker than the authorities. The cop couldn't read the kid's mind and probably didn't even realize the kid's age. His job is to protect the players and other personnel involved in the game and I applaud him for his efforts.
With that I leave you with this gem from the ESPN article on the issue:
"Several Phillies placed gloves over their faces and appeared to be stifling laughter at the wild scene."
I would have been laughing too. Idiot.