These police are absolutely deranged. Criminals aren't necessarily smart but they would obviously see that it would be easier more effective to use something less expensive than a gnome. Faster than placing a gnome. More Covert than placing a gnome. More likely to be adjusted/removed when noticed than a gnome.
Like what criminal would order/make a bunch gnomes when they could go to a grocery store and pick up 1000 flyers to leave on people's doorsteps. Worried about them blowing away? A pocketful of gravel.
Granted: a person on vacation might ask a neighbor to collect their mail, so burglars may miss some marks (false positive) but that is way better than robbing an occupied home because they left their gnome in place (false negative).
Also you don't have to explain to anyone why you're walking around the neighborhood with a bag full of fucking gnomes.