Good news, cops don't put people in jail. That's what courts do. You know "right to remain silent" and all the jazz?that's your opinion, but it's a fantasy.
It's not how the law is written. I don't want to live in a world for cops have blanket authority to imprison people over jokes.
Going to jail is no small thing, especially for a 15 yo.
It works like this*:
1) Law: You can't threaten people.
2) Person: Threatens people.
3) Cops: Arrest person.
4) Person: Goes to court where lawyers argue whether or not the person is guilty of breaking a law (this is where case law comes into play)
5) Jury: Decides whether person is guilty or not.
6) (a) If guilty: Judge decides penalty.
(b) If not guilty: Person goes home.
-> If 6) (a) occurs Person may go to jail and serve out a sentence.
How laws don't work:
1) Law: You can't threaten people.
2) Person: Threatens people.
3) Cops: Arrest person, skip all the due process and decide for themselves whether or not there is criminal intent then send people to jail at their discretion.
*This is a general outline, in some instance people go directly before judges rather than having jury cases, yadda yadda.