Making a product legal increases its availability, affordability and reach. So, the number of people exposed.
Really, a prohibition argument made with a straight face?
North American governments famously failed to maintain a ban on alcohol. How well do you think they'd fare with trying to ban sales of caffeine, given it's reach is greater (and far more socially-accepted) than alcohol?
Don't forget the vast selection of caffeinated pop/soda, energy drinks, etc. You're talking about a wide ban here, as this is one of the world's favourite drugs, covering a very popular array of beverages. If you're daft enough to propose it, I suspect you'd be laughed out of the halls of legislatures.
Regulation of caffeine levels in energy drinks and pop is probably an easier sell, and some jurisdictions already do this.
You don't get an addiction to a substance that has never been near your body.