When a window is in fullscreen mode, the user has the option to hit the meta key and reveal the taskbar. If the user clicks the fullscreen application (Currently selected), the taskbar should minimize that window. Currently, it takes the user back to the fullscreen window. This behavior is not consistent with behavior for non-fullscreen windows. The workflow needs to be consistent in spite of the window geometry.
It's because a window only gets minimized when its task is clicked if it's the active window. When you hit the Meta key, Kickoff opens, and from KWin's perspective, Kickoff is now the active window and the window is not active. So clicking on the icon makes the window active, rather than minimizing it because it was already active. You can see this outside of full screen window mode too. Just hit the Meta key and the background of the active task goes into "inactive" state.