I’ve been pondering this, and I can imagine situations where not having a round in the chamber would be desirable.
The first situation is while inside a friendly area, such as a camp, when performing routine duties, such as vehicle maintenance, carrying loads, etc., where a rifle gets in the way. In this case, having a rifle and bullets, nearby, or carried, but you are on support duty.
Second is while being transported in a vehicle, where an accidental discharge could be catastrophic. In a helicopter, for example, rifle barrels are facing down. The same with unarmored troop truck transport, with lots of vibration and bumps.
Third is a rifle that is prone to accidental discharge. The old M-16 was very prone to this. If the trigger was ‘pulled on’ with a round in the chamber and the safety on, it had a habit of, when taken off safety, to discharge a round without an additional, full trigger pull.
The current rifle, the M-4, is very similar in basic design, to the M-16, so likely still has this problem. This is the type of malfunction that would happen a lot while on a patrol.
Fourth is if the patrol was near to an area where non-hostile weapons were being discharged. As a rule of thumb, before a patrol, it is a wise idea to test fire weapons, to insure that they, and the ammunition, are functional. This is part of the regular checks done before a patrol, to include radio tests, etc.
But this means that other soldiers will hear gunfire, and not know that it is not hostile. If they are able to immediately fire their rifle, they might do so. (Which is why it is good to do test fires in an area out of direct line of sight, hopefully in a ground depression.)
In any event, the possibility exists that the soldiers themselves see the reason for not having a round chambered. Combat soldiers are not shy at all in complaining if they think they have been put at unnecessary risk.
The only reason I can think of that makes sense as to why the soldiers or their officers made this decision is that the “hearts and minds” thing over there is so delicate at this time that any accidental discharge killing an Afghan civilian could ignite a firestorm or a coup. And we over here don’t really know what the conditions over there are for those patrols. Since they at least have a loaded mag in place, it’s really not as big an issue as it might seem. Now if they were really on patrol with unloaded weapons, or carrying no ammo, that would be stupid and fatal.