Having been a member of a police SWAT team for about 8 years, we were taught to carry our SMG's at the "ready" position, which is where you are seeing these troops carrying their M-16's. By carrying the weapon in this position, if you're confronted with an immediate threat, you only have to bring it up a small distance and fire. After much training with carrying the weapon this way and firing it from the ready position, you get to the point where you only have to bring the weapon up and fire off
2 or 3 quick rounds without aiming. This type of carry also is a great advantage in urban combat where you work in houses or buildings where threats often do come without warning.
I know SWAT teams have been doing this for many years, though I was a bit surprised to see military troops carrying their M-16's this way, as the M-16 is a bigger weapon than the Colt SMG we used to carry.
I also believe, I may be mistaken in this though, that this type of carry was learned from the Israelis, who, like it or not, are the masters of urban warfare. They have taught many police and military trainers from around the world.