The First Amendment to the United States Constitution prohibits the making of any law ... interfering with the right to peaceably assemble, ...
As with Free Speech - if WE do not retain the RIGHT to peaceably assemble ... anonymously ... we will have indeed lost that right.
Being a passenger in a car stopped for a traffic offense is NOT 'probable cause' for an illegal search, which is what this demand for ID is.
Sorry - the ACLU is correct on this one.