I can't imagine any scenerio where it would be in my interest to aid Law Enforcement in the first place.
How about because he needs a hand.
How about because his family might appreciate it.
How about because it doesn't have to be in your interest, but can just plain be the right thing to do.
Police officers do many things everyday that aren't in their interest. Some would say that's what they get paid to do. Is getting/giving gas to a stranded motorist in their interest, is giving directions to a lost motorist in their interest, nope but they do it because it's the right thing to do.
Maybe some wouldn't stop and help, but I have and will in the future if the need arises.
From a personal experience I had in 1987, an IL State Police Trooper paid for my gas and accepted my check after the clerk called him because the Oasis in Chicago doesn't take checks. He came, I explained that I had just driven across the country on leave, I couldn't see their little sign from the pumps and was an hour away from home only to have the cops called because I didn't have $5.00 cash for gas. He talked to the cashier, she was rude to him, so he paid her and took my check. It wasn't in his best interest, but he felt it was the right thing to do. The best part was listening to her bi#$% at him because I blocked the pump island for an hour waiting for him to get there. I explained to him that I didn't want her to claim I tried to drive off, he thought it was a good idea gave me a wink then we went outside and talked about dogs and guns for another 45 minutes at the pump. To this day I still remember Trooper Love (forgot his first name) and how he did something for me that wasn't in his interest, and definately wasn't his job.