These guys aren’t in some kind of parade or ceremonial display, nor do they appear to be at a guard post.
They’re just soldiers standing by. In that situation there’s no need for a uniform display to impress anyone. Keeping soldiers at drill precision 100% all thetime is a good way to fatigue soldiers who don’t need to be perfect quickly.
Besides which, all of them have a hand near their trigger guards and most have hands resting on the barrel of their weapons so are reasonably prepared if someone does try something.
I too was infantry. And muscle memory doesn’t necessarily mean always being engaged to the process, it means being able to return to the necessary posture when needed.
It wasn’t uncommon for us to rest our hands on our rifles too when we didn’t need to be actively engaged in the current situation in any hostile fashion.