Karl if you would have went to the link I provided they have a picture of the sign. It cites article 18 section 930 and they do not site the entire statue.

The very first sentence of the statue says "Except as provided in subsection D"

They of course don't have subsection D listed on that poster because it clearly states that if you are lawfully carrying incident to a lawfull purpose it is not illegal to carry a firearm on federal property.

Go to the link I provided and read it a lawyer does not make law but they have to inturprit it that is their job.

Here is a picture of the sign notice it does not contain the entire statue.