Combat is not bench rest shooting at the range. You don't finish up and go check the bodies to see who got the tightest group with the fewest shots. In addition to what armysniper said about it possibly being an order to lay supressive fire, which is true, it also comes down to this:
Expend a moderate amount of ammo-->Enemy may live, and shoot my buddies
OR
Expend a lot of ammo-->Increase chances of killing the enemy, and saving my buddies lives.
Heck yeah I'll burn a couple 200rd drums of saw ammo if it means the squad tasked with maneuvering has even one less insurgent to deal with. There is always more ammo. This isn't WWII where whole divisions are scrounging just to find enough ammo to give each rifleman a few stripper clips. It's likely that this was a patrol out of a COP where there is plenty of ammo. Blow it off while finishing the mission, get more for the next mission. Lives trump ammo any day. I'd shoot 200, 2,000 or 2,000,000 rounds of SAW ammo into a hillside if it means my buddies have a better chance of not getting killed on their mission.
Also, Taliban like to spread out. The blend in well, shoot from a distance, rig up their own machine guns to fire by wire. This looks like an ambush where a handful of insurgents are dug in fairly well in the hillside, spread of pretty far. They grew up in this country and they know how to shoot, move, and escape quickly. If you want to see someone trying to take well placed individual shots or small burts, look of Designated Marksmen videos. SAW gunners are there for support through "volume of fire". That means putting a lot of rounds around the enemy, to pin them down, cut down on their cover and concealment, and give other soldiers a chance to move in for the real kill. Or depending on their air/artillery support situation, sustained fire can keep the enemy pinned down while air support takes them out.
I don't think any Americans died during that engagement. And that means his shooting was just fine. And notice they do start getting stingier with their ammo as the run lower.