This is a legacy issue from ArmA 2.

When you look at a nearby smoke screen with a scope, the smoke particles get smaller and you can see through the cloud of obscuring smoke. This only works for a few meters in front of the scope, meaning that smoke is highly exploitable. You can deploy it in front of yourself and use it like an invisibility cloak, using optics to kill everyone beyond it with impunity. This is especially heinous with vehicles, which always have access to large amounts of smoke and scoped weapons.