Relevant XKCD comic.
Let's assume that there is a cure for cancer. This is a huge thing, possibly even bigger than a cure for AIDS. If you can cure cancer, then you can pretty much name your price and people will pay it -- what are they going to do otherwise, just give up and die?
Oh no! The FDA won't let you provide your cancer cure in the USA! What are you going to do?
Move to Canada. Or France. Or Finland. Or wherever. I guarantee you that
some country out there would be very interested in playing host to the person who developed a cure for cancer. We're talking serious international prestige here, not to mention gigantic tax revenues. And those people suffering from cancer will just have to fly out to meet you before paying your price to get it cured. Again, what else are they going to do?
This is why the "A major problem in our society has been solved but the solution's being kept under wraps" stories always fail. Solutions to big problems are worth big money;
someone's going to find a way to profit off of them.