I don't know where the word "defense" first got inserted, if it wasn't
in the legislation's title itself. Maybe it was in the language of
the law. Not inclined to read it.

I can't blame you, since it would take nearly five minutes to find a
copy online and read it.

http://www.nolo.com/legal-encycloped...ghway-act.html

(excerpt)

Sec. 108. National System of Interstate and Defense Highways.

(a) Interstate System. It is hereby declared to be essential to the
national interest to provide for the early completion, of the National
System of Interstate Highways, as authorized and designated in
accordance with section 7 of the Federal-Aid Highway Act of 1944 (58
Stat. 838). It is the intent of the Congress that the Interstate
System be completed as nearly as practicable over a thirteen-year
period and that the entire System in all the States be brought to
simultaneous completion. Because of its primary importance to the
national defense, the name of such system is hereby changed to the
National System of Interstate and Defense Highways. Such National
System of Interstate and Defense Highways is hereinafter in this Act
referred to as the Interstate System.

So it wasn't the title of the bill, but the name was in the bill.

R's,
John