No one has to show a passport as long as your ticket is going somewhere in the US. Even the state licenses that don't meet the federal standard have been grandfathered into it for a few years, so they can fix it at the state level rather than pissing off voters.
There was a lot of fuzz here in Italy some ten years ago about Ryan Air not accepting as ID some particular forms of ID cards because of some fine interpretation of the Law.
In Italy a number of documents (driving license, boat driving license, other kinds of government issued ID cards, such as police ID cards, gun carrying permit, or similar) are considered as substituting the national ID card for the identification of someone, but for taking a plane only the national ID card (or a passport) is considered valid.
From Ryan Air site: https://www.ryanair.com/gb/en/useful-info/help-centre/terms-...
>INTERNATIONAL TRAVEL - TRAVEL DOCUMENTS ACCEPTED :
A valid passport (Note: all non EU passport holders, travelling into a Schengen member country are obliged to ensure that their passport is valid for at least 3 months from the date of their departure from the Schengen member country. This requirement does not apply to holders of a Schengen issued residence permit or long term visas).
A valid National Identity Card issued by the government of a European Economic Area (EEA) country. (Only the following EEA countries currently issue National Identity Cards acceptable for carriage on Ryanair flights: Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Estonia, Finland, France, Germany, Gibraltar, Greece, Hungary, Italy, Latvia, Liechtenstein, Lithuania, Luxembourg, Netherlands, Malta, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland. The validity of French National ID cards (issued for adults) has been extended from 10 years to 15 years ONLY for cards issued between 2nd January 2004 and 31st December 2013. ...
The request for passport probably derives from the UK seemingly not issuing a suitable national ID card.