More than once Nick is told to "quit being a cop and be a Grimm". The wessen know that some wessen crimes cannot be resolved by human law. The Portland wessen have learned that Nick is not the Grimm they were warned about as children, he doesn't kill every wessen he sees, only those who commit capital crimes and cannot be dealt with any other way. I may just be suffering from old timers disease, but I don't remember any wessen who could be convicted by human law that Nick just killed to keep his skills up. Ken could not be convicted because what would be a believable motive for some European coming to Portland to wipe out a bunch of neighbors and cutting off some woman's head? A serial killer of women found dead in a warehouse dealt with by a bunch of vigelantes-okay, that would do. This is not a story about normal life but a hidden life that no normal human would believe-Josh for example.