This is what I fail to understand - why on earth is someone who has threatened mass murder not deemed to be an imminent threat?!
Well, imminent means right then, like within a day or two. This happened a year later, so that isn't imminent. When a person first says it, they may be considered to be an imminent threat and held, but you can't hold them forever. You hold them until the mental illness has improved. And if they don't actually have a mental illness, you can't hold them at all. You have to notify law enforcement.
I hope moderators will allow me to post examples even though they're not from this case. I just think this question is asked a lot, understandably, because the way the mental health system works can be confusing. So if mods will allow, check out the examples below:
Scenario A:
Patient: I'm going to kill my neighbor, John Doe!
Doctor: Why are you going to kill John Doe?
Patient: He's working with the CIA to tap my phone so they can transplant memories in my brain everytime I call someone. They even talk for me sometimes. It isn't even me talking. They also follow me around town so they can map out my movements because they're planning to abduct me and completely wipe out my memories.
Doctor: I see. So how do you plan to kill John Doe?
Patient: With the gun I just bought.
Scenario B:
Patient: I'm going to kill my neighbor, John Doe!
Doctor: Why are you going to kill John Doe?
Patient: I just don't like him. He's always got people over, they're very loud and I swear, he's the one who dented my car last week. The SOB didn't think I'd know it's him, but I do and he'll get his.
Doctor: How are you going to kill him?
Patient: With the gun I just bought.
Scenario C:
Patient: I'm going to kill my neighbor, John Doe!
Doctor: Why are you going to kill John Doe?
Patient: It's a personality conflict. I just hate him.
Doctor: How do you plan to kill him?
Patient: I don't know. I just want him to move out of the neighborhood that's all.
Doctor: Do you own a gun?
Patient: No!
Doctor: Have you ever harmed anyone before?
Patient: Of course not! I get mad sometimes and say things, but it's against my religion to every harm someone.
Doctor: Well, you just said you want to kill your neighbor.
Patient: I didn't mean actually kill. I just can't stand him. I need help to deal with it.
The patient in scenario A would be committed against their will and likely treated with medication. They'd be discharged once the delusions and paranoia improve and the homicidal thoughts have gone away. This is the only patient who can legally be held because the homicidality is directly related to mental illness.
The patient in scenario B would be referred to law enforcement and under something called the Tarasoff Law, the doctor would have to notify the neighbor since the neighbor was identified by name as being the target of this patient. This patient's homicidality is not going to improve with medicine. It's likely criminal rather than mental illness.
The patient in scenario C would not be committed, but would be encouraged to engage in therapy (and maybe medications) to work on distress tolerance and conflict resolution. This is different because the patient recognizes that he's not really going to kill the guy. He's being provocative and temperamental, but the factors protecting him from doing it is that he's asking for help, he has insight, he doesn't own a gun, and he's religious.
There's more that goes into it, but that's kind of a superficial gist of why some people are held against their will and some are discharged.