I thought it was impersonating a federal agent in the line of duty that was a crime, ie it would not be a crime to claim that you work for the USCIS at a party,

I thought it was impersonating a federal agent in the line of duty that was a crime, ie it would not be a crime to claim that you work for the USCIS at a party, but it would be a crime to claim you work for the USCIS and demand to see someone's green card.