They may have decided that it's economically smarter to let her go and let the other girl stay. Which is poor child development, but sound finances. Bah.
Well, it's probably more complicated than that. They may be weighing a bunch of factors. When Franny was in preschool, she had a classmate with significant behavior issues. Not just hitting, scratching and biting, which are problems, but developmentally they aren't out of the ordinary. He peed on other students, peed on Franny's clothes, threw food at her regularly, etc. Parents complained (including me of course), teachers complained, but they tried to keep him enrolled. The school clearly was motivated by benign intentions, but in the end I think everyone felt a bit betrayed. Parents like me who felt the space wasn't entirely safe for their children, and the parents of the boy who was finally asked to leave.