But turning an off-label use into an on-label one to promote it has its own nasty consequences, even if the drug is proven to work for the new indication (or whatever).

Consider Lucentis vs Avastin. Ophthalmologists were using Avastin off-label for wet AMD. Genentech was making a huge multifaceted effort to discourage off-label use because they had sunk a bunch of effort/$ into the order-of-magnitude-more-expensive Lucentis for not much of an improvement. Similar situation and different ending with Lumigan (glaucoma/ocular hypertension eyedrops) vs Latisse (eyelash enhancer), where Latisse's patent (afaik) was thrown out because Allergan was trying to patent a known major side effect of Lumigan.

I'm not a fan of misleading promotions of off-label uses either, but I wish there was a middle ground still banning that that didn't also involve more patents and protection.