Why are building entrances unlocked during the day?

Many campus residence halls have classrooms and administrative offices. In these buildings access to those spaces are open to the public from 6:45 a.m. to Midnight. However, the living spaces in these halls are secured 24 hours a day by access doors that are not open to the public and require an access key to enter. Halls that do not have public spaces are secured 24 hours a day.