Why tourists have to stay outside the gate of the White House

My cousin was my tour guide to DC, in the spring of '94.  This classic, impromptu, photo was taken, after he did the jokingly, I'm going to jump the fence routine.  After the flashback, and taunting on my part to try and get him to do it.  During my childhood, I had witnessed several protests at the Bangor Naval Base, north of Silverdale, WA, along little, ol' Clear Creek Road.  When the protesters jumped the fence, it was a mad-dash, trying to out run the working dogs, military security, and anyone else along for the ride to tackle the trespassers and get them on the arresting bus.  It would be a long, hard drag across grass and asphalt, until the trespasse was stuffed on the bus.