Probably because they want to reach the largest segment of the buying public. Mopar lovers will buy anything that's a little unusual or out of the ordinary. Having your very own "Hemi Police Special" would appeal to more people than you might expect.
In the one photo of the car, it looked like he had a Ford police car parked right next to the Dodge. Maybe that dealer specializes in offering rare and unique rides to his customers.
On the other hand, Police departments shop EBAY too. My cousin is the captain of a local force, and they sell stuff on EBAY all the time.