Just because it's a public building doesn't mean anyone can enter any area at any time. Case in point, the Oval Office is a public building -- try entering it uninvited. See how far you get before you're shot Ashli style.
So much for an open democratic government working for the people. The police state rolls on and some Americans have no problem with it.
Congress is a unique building. It is both a public building, that has within it Top Secret restricted areas. Which means entry into those restricted areas can be repelled by the force necessary to prevent entry.
In addition the area was an area being used to protect the members of congress from outside harm. Making it not just restricted by statute, but restricted by order of the secret service.