For those that do worry about these things Under 8 USC 1304 (e) a resident alien MUST carry thier I-551 "(e) Personal possession of registration or receipt card; penalties Every alien, eighteen years of age and over, shall at all times carry with him and have in his personal possession any certificate of alien registration or alien registration receipt card issued to him pursuant to subsection (d) of this section. Any alien who fails to comply with the provisions of this subsection shall be guilty of a misdemeanor and shall upon conviction for each offense be fined not to exceed $100 or be imprisoned not more than thirty days, or both"
So yes, by Federal Law they must be able to "Show htie rpapers".
AS far as the stunt, pretty mindless.