Its simple.
Cops have level 2a/3 armor.
No mainstream pistol round, to my knowledge, can go through level 3 armor. 44 magnum is the only one to my knowledge that can pierce standard 2a armor.
Some mainstream rifle rounds can.
Less rifles to deal with= easier time come confiscation time.
Level III does not mitigate .30-06 or 7.62 x .54R round fired at 2800 feet per second and non-steel core. Weapons that utilize these rounds are not Assault Rifles. For now they are mainly assigned to assaulting the local deer population.