Even individual liberties have limits. They are set by things called laws. But you knew that.
What particular law applies here? I'm sure there are quarantine laws that apply to those who actually have certain diseases, but in this case they're quarantining based on the possibility that someone might be carrying one, and I'm not too sure any such law exists, else they wouldn't have let her have her way, and they wouldn't be scrambling about trying to find some legal means of keeping her isolated.