A big part of it is likely that they didn't know if the National Guard would be called out if things got out of hand. I believe the police let them in because t

A big part of it is likely that they didn't know if the National Guard would be called out if things got out of hand. I believe the police let them in because they were outnumbered and thought letting them through once the premises were evacuated would prevent an even worse outbreak of violence if they (the police) were overwhelmed by the crowd.