I understand your frustration... I was looking for something in your story regarding OC but was let down when there was nothing.
The fences are more for night time visitors. DC had problems with muggers and robbers walking thru the area and the fence is a way to limit where people go.
Keep in mind that there are several police agencies there. DC, Metro Authority, Park, Capitol, FBI, Homeland Security, and USSS. I am not sure what ones you saw there that day.
Those agencies have little to do and will be seen all over the area. They are a show of force so that the terrorists know that they will meet heavy resistance if they tried anything in DC.