I find that the cops and robbers concept is more relevant to me and my kids than the military concept. We watch a lot more police drama than military, and the police are in the news everyday it seems.
The concept also let's players explore the police experience and learn about alternatives to deadly force. My son is nine, and we talk a lot about self-defense, bullying, and tactics. He is aware of the necessity for less-than-lethal approaches in policing, and Hardline helps him understand this with its various scenarios. I think it is a great educational tool when teaching about bullying and the deadly force continuum.
For me, it saved the Battlefield franchise because I find the other stories boring!