Well the M14 platform vents all the gas downwards out near the front of the gun anyway, so the gas wouldn't be in the shooters face. From what I understand, suppressors, even with supersonic ammo, really help to de-localize the report of the shot, as well as acting as excellent flash suppressors. They might have turned off the gas in order to stop the action from moving and throwing a nice shiny piece of brass 10 feet into the air and giving away their position that way.
I seem to recall reading that some of the US snipers were displeased by the lack of forward assists on the newest generation of AR based sniper rifles because it meant they could not slowly charge the rifle - they had to do it quickly, which could give away their position. Same idea maybe?
