If inzomnia is mafia her job has no purpose; she would already know who was and was not mafia although s.a.m.'s job isn't much better
Even if she has no purpose, she is still mafia. And getting rid of them is our number 1 priority. We shouldn't risk arresting someone we aren't 100% sure of. Not that I am 100% sure of s.a.m., but I am more convinced of her than anyone else at this point.