National defense industries are exempt from patent laws -- this is fact. They also don't publish their findings (i.e. they don't apply for patents), because doing so would also violate national security act. They're free to use any patents they want without royalties.
In other words, they're free to use commercial patents without royalty and they dont' publish their work on public patents like commercial entities. When you get a patent, you HAVE to disclose how it works, so by disclosing it, you're giving away secrets.
It is a fact they are not exempt.
"(...they don't apply for patents)
Aerospace and Defense companies have vigorous patent filing programs complete with patent counsel (or, as they are usually called today, "intellectual property counsel"). Patent filings are routinely made.
"...doing so would also violate national security act."
No it wouldn't. The procedures for filing classified patent applications is well understood by company patent counsel.
Where on earth did you get your information? You're spreading a lot of misinformation.