“Center caps”. I see. Thanks for all the info, everyone!
Interesting. I guess times are changing. I should have said “usually don’t have aluminum rims”. Around here (Dallas) I can’t recall ever seeing a police car with aluminum rims.
Post #2, too. 
It is not fair to everyone when you insert a post like that after the fact.
It’s been answered: because they go flying off during high speed cornering.
The worst offender of all time was the early 90’s model Chevy Caprice. Take a sharp corner at a higher rate of speed and DING…ding…ding…ding…ding! Suckers would fly off across the road. Happens on Crown Vics too, just not as bad.