Screens are requirement by state of Calif., but also necessary for locks too:
Landlordâ€™s Duty to Provide Security
â€œEffective July 1, 1998 landlords or property managers of all residential rentals in California must install and maintain operable dead bolt locks on all exterior swinging doors that provide access to the living area and must install some type of lock that complies with the building code on all exterior doors that provide access to common areas in multi-family developments. This same law requires operable locks on all windows unless the window is more than twelve feet off the
ground. It does not cover mobile homes, but does cover vacation rentals.â€
Hope this helps.