FDA approval can take years and millions and millions of dollars. Last year Restylane finally received FDA approval for use in the lips, however, most physicians preferred that injectable in the lips for years before it. Sometimes the usage of a product leads the company to go back for further FDA approvals.
It's not illegal or unethical for a physician to use injectables "off label". Botox is only FDA approved on the face for the glabella, however, most physicians use it on the forehead and around the eye area. Most patients would be unhappy if we said we'd only treat the glabella region because that's all that's on record for FDA approval. However, there is no reason not to treat the forehead and other regions - as long as you know what you're doing!
There is nothing to say that "off label" usage is harmful. What's harmful is when inexperienced physicians or worse non-physicians inject things they don't understand - that's dangerous! "Off label" usage by experienced practioners isn't.