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