You might point out that after the London Tube bombings, the police were begging everyone with a camera or cell-phone camera to please send their pictures in, that it might help them identify the bad guys. So the British, who have a lot more experience with domestic carnage than we do, seem to have the opposite attitude. (Just don't photograph children in the U.K.)
There seems to be some ambiguity about whether there actually is a legal ban or not, but it appears that last year, the MUNI authorities were acting as if there was, but could not cite a specific law when asked.
http://www.sfist.com/archives/2005/02/14/homeland_insecurity_photo_ban_on_muni.php
http://www.sfist.com/archives/2005/02/15/muni_photography_ban_update.php
http://www.shooter.net/index.php/weblog/Item/attack-of-the-sf-muni-fare-inspectors/
etc. These incidents happened a year ago.
Just Google these words: photography ban BART San Francisco
You'll get lots of hits for your reading pleasure.
--Peter