Panda bites third intruder in Beijing Zoo

A Beijing panda by the name of Gu Gu has just tasted human blood for the third time. Three men have in the last two years entered the panda's enclosure for different reasons, and every time Gu Gu has responded by attacking with dire consequences. I just wonder why the hell people think it is all right to do that? Is this a common occurrence in zoos all over?

I can't really say that I feel terribly sorry for the intruders (of which the first - a drunkard - bit the panda in the back, even).