Why do they feel a need to restrict them?
I don't have to justify a need. But, if you're trying to talk to a truly curious fence-sitter, here's some reasons.
- Millions of veterans want guns like the M4s/M16s they used in the service. The M16 has been fielded for five decades so there are a lot of vets who have defended freedom with America's rifle.
- Vets and civilians have appropriated military designs since at least the cowboy days (45-70 gov't round still in use today), WW1 (the now extremely common 30-06 and bolt actions -- many Fudd deer rifles are patterned on Mauser actions which the US actually copied...), WW2/Morea (again the '06 in the M1 Garand), the M1A/M14 (early VIetnam), and now the AR.
- Armalite Model 15s are easy for anyone to use. With an adjustable stock they can fit a small woman or a large man. They are light enough for a smaller person to handle easily.
- Patrol rifles are the #1 rifle of choice for police officers these days. They use them because they are very good for delivering controlled, aimed defensive fire at a bad guy. Civilians like them for this reason as well. They are the rifle of choice for shooting bad people in the thoracic cavity and face.
- The 30 round magazine is excellent for a civilian who grabs their rifle at 3 AM in their underwear to confront multiple heavily armed home invaders. Bad guys often take multiple shots to stop and the home owner in their underoos likely has no reloads, whereas the home invaders brought as much ammo as they wanted.
- The low powered 5.56 round is a great home defense cartridge. Based on FBI testing, it has less lethality after passing through a layer or two of drywall than a handgun or a shotgun buckshot round. It does not recoil very much making it usable by anyone.
- With modern ammunition or an easily swapped barrel (really an upper, but they won't know the difference), they are useful for hunting. Lots of folks out west hunt prairie dogs, coyotes, and other varmints with them and like having a standard capacity magazine for that.
- Because "F You." Refer to sentence number one and two above.