The funny thing is her parents were upset with what the Secret Service did but probably aren't the least bit upset with their "darling" little princess for making the threats.
If I was a minor I don't think I would want to be questioned by anybody without my parents around. That said, I'd be more afraid of my mom and dad than I would be the police.
While this girl is probably guilty as heck, there is such a thing called due process of law: innocent until proven guilty. Keep in mind the possibility that it is very easy for someone else to pose as you online, and post things that would get you in trouble. The FBI uses this technique (i.e. creating a false identity) to catch child predators.
The parents should have been called in and questioned as well. If they didn't know what their little darling was doing, they know now. They need to learn a lesson, too.