Curiosity
Just out of curiosity, what would have happened had a visitor to the UK made a threat against a member of the royal family? Would they be detained? What does the law say?
Would it be any different if the person was a male or female? Young or old? A different enthicity perhaps? Catholic, Protestant, Buddhist, Muslim, Atheist? British subject or foreign visitor?
None of these factors prevents one from doing something stupid, nor do they make one more likely to do something stupid.
Here in the US, while we do have the freedom of speech, that freedom does not extend to making threats. If there is a law prohibiting something, the authorities are required to act. Should they fail to act as the law dictates and something untowards occurs, they are held liable.
I'm not saying that she (not a young girl, but a woman) should be jailed for 6 months, but she has the right to trial (a right guaranteed to everyone here) by a jury of her peers. From an american perspective, she's got nothing to worry about. We're letting criminals off with a slap on the wrist because of the tendencies of the average person who sits on a jury!