Comment 5 for bug 845856

<https://wiki.ubuntu.com/MenuBar#Presence>: "The menu bar should appear at the top of every screen, except those screens where a window is currently in full-screen mode."

<https://wiki.ubuntu.com/MenuBar#Windows_with_recognized_menus>: "Where the active window has its own menus in a menu bar in a supported toolkit, these menus should be presented in the menu bar..."

So, it's not a design decision to put the menus on one display or another. They should be available on every display that has a menu bar.