Quovatis, i agree with you and am not arguing that they should be able to shoot over walls. This topic is about tanks though.
The only argument i could give for machine guns to be able to shoot over a wall is, "They climbed the walls to shoot", but in that instance the machine guns should have to stand right next to the wall to 'consider' them climbing the wall.
In all reality, only mortar, cannon, helicopter, aircraft should be able to shoot over walls, and cannons shouldnt be able to shoot over at close range.