This Federal law only has jurisdiction in federal holdings with reguards to possession, so the exception can only be applied to where the law is enforcable. The wording is not granting a federal protection as much as it is allows a condition where the federal law does have an exception.
Read the law with what I explained and I think you will agree:
Section 1243. Manufacture, sale, or possession within specific jurisdictions; penalty
Whoever, within any Territory or possession of the United States,
within Indian country (as defined in section 1151 of title 18), or
within the special maritime and territorial jurisdiction of the
United States (as defined in section 7 of title 18), manufactures,
sells, or possesses any switchblade knife, shall be fined not more
than $2,000 or imprisoned not more than five years, or both.
Section 1244. Exceptions
Sections 1242 and 1243 of this title shall not apply to -
(1) any common carrier or contract carrier, with respect to any
switchblade knife shipped, transported, or delivered for shipment
in interstate commerce in the ordinary course of business;
(2) the manufacture, sale, transportation, distribution,
possession, or introduction into interstate commerce, of
switchblade knives pursuant to contract with the Armed Forces;
(3) the Armed Forces or any member or employee thereof acting
in the performance of his duty; or
(4) the possession, and transportation upon his person, of any
switchblade knife with a blade three inches or less in length by
any individual who has only one arm.