In the "View" menu, in the "Side pane" submenu, there are checkbox that allow to select the side pane view. Checkboxs are used when you can select more than one option, in this case should be radiolists since you can choose only one view, using checkboxs in this case would be confusing.
It's the same as firefox does, because a radiolist won't work for two reasons: (a) the "no side pane at all" case and (b) the keyboard accelerators. There should be an old bug describing this problem, but I cannot find the number right now.