The design is clear and intentional.
This design says that the Magic Mouse is not a cabled mouse. It is a wireless mouse.
This design says, one night a month, put your mouse up on its side to charge it, because charging your wireless mouse is a discrete action.
Imagine optionally driving a Tesla while plugged into an outlet(with a loooooong extension cord).
Were the port in a place that allowed operation, how many people would keep it plugged in? Probably more than Apple designers could stomach. Remember this is a wireless Magic Mouse.