Take control of an infantry soldier
Select your primary weapon and use the 'adjust stance' keys to move your avatar up and down, left and right. Notice your avatar leans left and right and raises/lowers its height incrementally.
Select binoculars and attempt to use the 'adjust stance' keys to move your avatar up and down, left and right. Notice your avatar does not respond to adjust stance keys.
Go prone while holding binoculars and press the 'evasive left' or evasive right' keys. Notice your avatar does not roll while holding binoculars.

Currently, players using binoculars present an easy target for enemy fire.
There is no realistic reason players should not be able to adjust their stance while using binoculars in the same way as they do while holding a weapon.