I'm betting there's an ulterior motive to being in Florida and she's looking for an excuse to stay.
She probably just really likes oranges.
On a more serious note, it could be a matter of pop-Christianity. Different regions grow different churches. In Kentucky, you get your typical baptists, but in Seattle, there are more and more non-denominational, and 'emergent' churches. I don't know what's down in Florida, but it wouldn't surprise me if some movement based out of there (or even the pastor that took her in) might be a major draw factor. I know I was ready to up and move to Vancouver when I was a young Christian, because there are a lot of churches and organizations there that appealed to my view of what Christianity should look like.
Since all I really know about the girl is that she's converting to Christianity, that's the only guess I can offer. Although, the oranges thing probably doesn't hurt.
Regarding the foster care and whatnot, yeah, it seems a bit foolish, if she's only got a few months anyways. What is the law down there on moving out? I'm fairly certain someone can live without a guardian at the age of 16 here, but foster care could go up to 18. I could be wrong. I only know of two examples which I'm drawing on, but I get the impression that's how it works.
If that's the case, and she could be on her own anyways, I would suspect that it's a moeny-grab. But if she can't live guardian-less or whatever for those months, then perhaps bringing it to court is the only way the state won't send her home for those seven months in between? I'm just sort of spitballing here.