Border patrol is the same way. I have no idea why they do it but they do.
One, because even in a plane you are subpoenable, not to mention you need to know the rules regarding law enforcement while you are flying (why on my drug ops deployment we had Coast Guard guys on a Navy ship and why we always flew with them, and why we couldn't share our intel with them as it became subpoenable after a bust, and we didn't want to burn our sources)
Two, if you put an officer in a plane, they are a cop first and pilot second which means they probably are not going to go to XJT after they get 1500 hrs. Good use of taxpayer dollars.
Spongebob