Their logic is so flawed...
If you want to just shoot for sport or go to the range and have a good time ammo costing 3x as much will probably make you think twice about going and even buying the weapons. So great, you took a guy that just wanted to have a little fun out of the equation.
If you want to kill someone, or a lot of people, I highly doubt you are going to care how much the ammo costs. Even if a bullet was $20 any slug with a pulse can get a credit card for a few hundred bucks. More than enough to shoot up the local post office. So woot, you haven't deterred the violent crime.
I dunno how far I see a lot of this paranoia going, but everything has to start somewhere.