It's raining here too, same area as you.
Brille already explained it but yeah using your tanks to kill things without seeing them.
For instance you have a tank with red optics, but you still use it to area target known infantry locatioms across the map. I'm sure you can imagine other situations where this is abused.
Theres no way to allow area fire but prevent people from snipinging infantry this way, so a hard rule - removing the feature for MP- would be the way to go.
It's the lesser of two evils.