They put this ban up, due to people taking potshots with sidearms at helicopters last night, during the protests.
As a St. Louisan, the social dynamics are much too nuanced to even try to explain, so I'll just say, it has not been good the last couple of days.
https://twitter.com/MatthewKeysLive/status/49926643621314150...
This is a local alderman filming: