firearms in a federal facility question

[unrequited]

(a) Except as provided in subsection (d), whoever knowingly possesses or causes to be present a firearm or other dangerous weapon in a Federal facility (other than a Federal court facility), or attempts to do so, shall be fined under this title or imprisoned not more than 1 year, or both.

(d) Subsection (a) shall not apply to—
(3) the lawful carrying of firearms or other dangerous weapons in a Federal facility incident to hunting or other lawful purposes.

http://www4.law.cornell.edu/uscode/h...0----000-.html

Why isn't carrying for personal protection or self-defense a "lawful carrying of firearms incident to other lawful purposes"?

[DefensiveCarry.com]

[Bob The Great]

2 words - case law. Basically, courts have decided that self-defense is not covered by "lawful purposes", in spite of what the written law says, and that trumps normal common sense.

[cagueits]

Because the law is vague and the judge that sees one case does not necesarily see it the same way as other judges may see it. So what is legal today may not be tomorrow, and then it may be legal again or not in the future.