C'mon, wake up. Any excuse they get to force protesters to show ID, is an opportunity for them to enter your name into their secret case files. Big Brother is watching. This is part of how they single out the most active protesters for greater harrassment later in their activist careers, as well as evidence they'll use against you some day if you're arrested for something greater like a "terrorist conspiracy".

It's just daily fact-finding for the surveillance state. They do it because that's how the system works, and because privately they see all anti-war protesters as enemies of the state.