That's almost true, and is a well known part of pharma strategy. For example Botox was initially approved for hyperhydrosis (excessively sweaty palms and armpits) and was used off label for its most well-known application, wrinkles. It was finally approved for wrinkles only recently having been approved for other things along the way.

Any MD can prescribe any (non-opiate) drug for any indication. But the drug companies are prohibited from doing what you say: telling docs about usages they haven't proven in trials. Sometimes the sales rep will drop off a scientific paper (often funded by the drug company) about some other use of the drug -- enforcement in this area is uneven.

If they do want to advertise new uses they need to do new trials. And though existing trial data can also be used, if the dosage regime is different or, say the patient pool tox risks are different (perhaps the new indication is less serious than the orphan one) whole new tox studies may be needed. Which seems fair.