It also means you give yourself away ‘cuz you can’t silently chamber a round.
The enemy will know you are there, just like chambering a round in your Glock or your pump shotgun.
Chambering a round has a VERY distinctive sound.
During some hunts even releasing the safety produces enough sound for the animal to notice you. Savage rifles are good in this aspect, but Weatherby - not so much, can't just flick the lever with your thumb; Vanguard, for example, just insists on clicking unless you very slowly guide the safety catch all the way forward.
Somehow, I'm guessing this rule does not apply to the tiny fraction of troops who actually have to worry about "giving themselves away." Or were you one of those ninjas who pulled all the velcro off his uniform because it wasn't "tactical?"
And what a great sound that is!