As a graduate of the Infantry School in 1983, the policy was to remove slings for use in the field. As a student in the 95th MP school later, slings were considered a weapon to use against the soldier - in riot conditions, grabbing the weapon - or riot baton - still firmly attached to the opponent allowed you to throw them off balance more readily.
Slings are to carry a weapon on your shoulder where it is difficult to use. MP's did use them when apprehending and searching a detainee on the street, as there is usually one other teammate positioned to shoot the individual if necessary, they aren't going to be able to hold another weapon for you. You sling it behind you, search, secure, and then regain it.
There has been a big influence to market and sell tactical slings, which Kyle demonstrated well. Knowing both the pro's and con's of their use is important, tho. Moving thru the field, in combat, to assume a firing position, a sling can be a major hinderance, and for hunting purposes I have rarely used a weapon with one. Same for urban use in tight quarters. In close combat, the sling isn't enhancing the weapon, awareness does - as said, the mind is your weapon.
Is this case a great example of why an AR is ok to use, it has demonstrated it's strengths. Not its weaknesses and the next situation may well do that. A 20" M16 is at a known disadvantage in close quarters which is why Colt modified it long ago - and that weapon was actually accepted for use before the M16 itself was adopted wholesale - the XM177 in 1965.
If one or the other rioters could have been muzzle jacked or buttstroked there may have been a different but similar story. A rifle has other uses besides launching a bullet as any Infantry soldier has been taught. You don't always have to pull the trigger, and at contact range it always boils down to close combatives. There is a reason for that lug under the front sight, too, and it is very much included in riot training. The term "arms" in the 2A is not about firearms exclusively, it's about all weapons, and in the day, they were primarily edged weapons.
