I have to admit, this is really clever. It walks the line between "too big to ignore" and "too small to properly prevent" perfectly. Guards won't shoot at twitchy little drones, because that's just asking for collateral damage. Fancy RF devices that could zero in on the controller or zap the plane would be too expensive and techy. Nets would be very expensive, over the areas they'd have to cover (unless it's a truly tiny yard) and even then, they could be overcome by an upgraded drone dropping heavy packages (or tiny ones). And on top of all that, it's cheap. Even if you lose a drone every other time, the drone is just the packaging for the much more expensive drugs you're dropping.
I don't see this having a happy ending--probably they'll just cancel outdoor time for everyone--but that's modern prisons for you.