This is the logical way of doing it. TileMap can have many layers(Object Layer, SpriteLayer, tile layer) so if you try to do
It should not reduce opacity of whole object which is logically correct. What if developer only wants to reduce opacity of Object Layer or tile layer and keep the rest to default? and if one wants to modify whole map opacity then they should loop around the layers and set them individually, giving developers more flexibility in managing their layers.