I'm thinking the fountain knew, in an animalistic instinct, as a solid, inanimate, object, it could eventually be overwhelmed by attackers and abused and corrupted. By being in the body of a person, however unwilling the person is, it can be moved to safety if there's an attacker, and better protected. Which is why the magi and guards are so evasive. Some know about this plan and feel bad for the young lady, but also know that the sacrifices done by one young lady, is the safety and survival of the magical world. One life vs many, many lives. A heartbreaking choice made, and all they can do now is try to ease the discomfort as much as they can for her.

At least that is what looks like to me, I could be wrong.