Why not? Cars claim more lives and are inherently more dangerous due to kinetic energy vs potential energy - essentially you're more likely to kill someone with your car than your gun because it takes active attention and constant decisions to control the kinetic energy of your vehicle. Frankly, we should be focusing on improving testing for drivers licenses to save lives. We are abysmal compared to most other developed nations.
Purchasing firearms is already regulated to varying degrees based on state. What restrictions are you proposing?
"If the lethality of a weapon doesn't matter"
It's about where to draw the line of the lethality of weapons allowed. Under the current laws there are a variety of restrictions/laws, but essentially non-automatic centerfire arms 50 cal and under is where that line is drawn. That could be redrawn again, but there would need to be discussion about where the new line would be and why.
And the real reason I wanted to comment but got caught up in those other replies... didn't the buyback miss a lot of weapons? I heard from a friend who lived over there for a while that many in the rural areas ignored the buy back and illegal guns are fairly common if you know where to look.