Because of the way Law Enforcement looks at an air/spring rifle vs. a black powder (primitive) weapon vs. a modern firearm.

The ammo is extremely light weight and one can carry a couple of thousand rounds (BB's or pellets) before the weight becomes noticable.

Air and spring rifles are fairly small and easily concealable and when viewed by another, looks much less threatening than most firearms.

That's My Honest Opinion, take it for what you think it's worth!