Whats not to understand,the cop saying that spoke volumes.
"The Thin Blue Line is symbolism that in Policing represents honor, valor, dedication, solidarity, and a host of other things within an honorable profession. It is used to remember the sacrifices officers have made throughout their careers. In some cases, that sacrifice is the giving of one's life."
Its not just some thing he saw on t.v. Theres a reason he thought it would work. Seems like it worked many times before since he assumed they would just take his word for everything.