When someone you know is accused of a serious crime, you immediately reassess everything you think you know about them. If you scarcely know them, you will probably go along with whatever the police and prosecution say. If you know them well, you may be surprised. You may even know them so well that you refuse to believe that the accusations can be true.
If you know them, well, – so, so; not too well, but more than a little, you are still likely to be swayed against them. The British girls knew Meredith quite well; Amanda, not so much. Some had never met her. In the circumstances, their behaviour is understandable.