It's a movie thing, seems to go back all the way to John Wayne in "The sands of Iwo Jima" or earlier. I'm a NOLOAD Marine with 3 years in the National Guard and my NCO's in either would have kicked my ass if I did that crap, same for roaming around with grenades hanging by the spoons from your web gear, might work for Mark Clark but he was a General so his were probably dummies.

I did have a buddy get killed in a Helo crash because his rifle drove his head back when they hit and it snapped his neck, they drill it into you in Boot Camp to make sure the rifle is clear of the helmet so that doesn't happen.
He screwed up and paid for it.