In the US it's against the law to threaten anyones life.
It's against Federal law, the highest and most severe, to threaten the Presidents life. The US Secret Service, tasked with protecting the President, investigates all threats aimed at him.
"Arrested" is a term where the person is being detained. If you are stopped for a traffic violation, your freedom of movement is temporarily suspended, so technically you were "arrested".
It's not like they put her in prison for the threat, which is technically against the law. They just questioned her and investigated the threat. They have to, it's their job.
|