Hehe, back in the 80's when I was 10 years old I could walk into the local wal-mart and buy shotgun shells in southern Missoui. (town population 2500)
I think the theory with felons is that offenders tend to repeat at some time or another. It's a generalization, not particularly fair, but that's the reasoning. So they decided to not make it easy for them to buy guns. It gives politicians the excuse that they "tried" to stop it.
Many of our laws today are more about emotion than logic. IE "Will someone PLEASE think of the children, PLEASE".