This is perplexing to me. I have never encountered this policy, and the
National Archives web site makes no mention of it("Show official
identification that includes a photograph. Acceptable IDs include driver's
license, passport, and school and employment identification."), or special
requirements for non-US IDs.
Ah! You should have talked to regular foreign travellers in the US; more
often than not non-US driving licenses (etc.) are not accepted. Passports
are the only widely accepted form of ID for most travellers. Especially in
the capitol.
YMMV, but I couldn't order drinks using my license the last time I was
there :)
Tom