The EFF notes:
Events with strong cultural, ideological, and political components are frequently held at venues that would be affected by these rules. Scanning the ID's of all attendees at an anti-war rally, a gay night club, or a fundraiser for a civil liberties organization would have a deeply chilling effect on speech. Participants might hesitate to attend such events if their attendance were noted, stored, and made available on request to government authorities. This would transform the politically and culturally tolerant environment for which San Francisco is famous into a police state.
The group is encouraging concerned citizens to attend a public hearing on the evening of April 12th. Find more details on this here.