So she was 17 when the age of consent here was 16. So that's fine. The law which applies is the law of the country in which the crime was committed - doesn't matter what the age of consent is in the USA.
She was trafficked - ok this may come out when Ghislaine Maxwell gets into court - so if this is proven, with factual evidence to support it which stands up in a court of law, then this is a crime.
But you then have to prove that Prince Andrew knew that she was trafficked and took advantage of this - if he can prove that he didn't know this, or the accusers can't prove that he did know this, and we are talking 'prove beyond reasonable doubt here', then it will not be a crime.
Given the length of time that has passed since the alleged incident, you can see why the Met have dropped this, given the % of rape convictions is about 1%, when such a long time has passed since the incident, and you also have to prove that he knew that she was trafficked (as opposed to knowing she was 17, knowing that she was above the age of consent in this country, and thinking, alright then). The Met won't be able to prove this beyond reasonable doubt in a court of law and so they have dropped the case.
It might not be ethical, it might not even reflect what did actually happen, but there has to be evidence to support alternative views, and given he's said all along he knew that she was 17 but didn't know that she was trafficked, 20 years on it's going to be extremely difficult to prove in a court of law anything other than he had sex with a 17 year old whom he thought was up for it.