Once the FDA approves of a filler (or any drug for that matter), a physician may use it for a patient in any way he or she feels is appropriate. If it is used in an area or in a way that was not specifically FDA approved, it is being used "Off Label". 40% of all prescriptions are Off Label. Off Label use of a filler is not illegal, unethical, or immoral.
Use of a filler that is not FDA approved is illegal, unethical, and just plain wrong. While in the United States, do not allow anyone to use a product that is not FDA approved on you. There is no need to do this except for the fact that the non FDA filler may be cheaper. Once the complications begin, the pain, the deformity, and the medical bills will make that acceptance of initial lower cost seem foolish.