Why does Mac OS X turn off monitor #2 when an application is maximized on monitor #1?
Why?
Personally,. because I don't think they ever thought it through properly. There's a lot about fullscreen that's like that. 

The fix is possibly System Prefs > Mission Control > check Displays have separate Spaces



The better fix is don't use fullscreen, use Alt/click on the green button to maximise instead.