[Queerasfck]

Besides the obvious reasons, it can be dangerous to be tasered, especially if you are under the influence as sometimes fans are. There have even been a few cases of deaths resulting from tasing.
A lot of security guards at games are not in shape to run around on the field to try to catch someone. But I think if there is no other way to stop someone make sure there are some signs clearly stating what will happen and if you're that stupid, silly or drunk to run on the field than you might get tased.
Also, I've seen this happen at a game. It's probably worse.