Well, the constitution and the bill of rights were written to replace the articles of confederation, which has resulted in a national government with next to no power. But I rather imagine a lot of people here would prefer it that way.
So, isn't lawfully carrying a firearm, for which I hold a state-issued license, for the purpose of self defense a "lawful purpose"? Thus, I believe that the widely cited 18 USC 930, if tested in court (and I cannot afford to be the test case) would likely NOT hold up if applied to a person lawfully carrying with a CHL.
I don't believe anyone has to risk arrest by walking into a federal building with an otherwise legal weapon. A suit can be filed to have the policy changed and signs removed, on grounds that personal protection is justified.