The sign instructs actually instructs you to increase “Smash and Grab” crime:
There’s no smash in the smash and grab if the vehicle is not locked. There’s only grab crime.
A resident of South City (St. Louis, that is) offered this advice: Don’t leave anything in your car and don’t lock your car. Otherwise, the baddies will smash the windows to get in. Whether they steal anything or not, you’ll be on the hook for a new window.
Basically, the trade-off is this: Locking the car will stop a casual thief, but a determined thief will have to break the window to steal.
(History proves that I have never liked cities spending law enforcement budget on these things.)