When a Mac app does need a user response before it can proceed, it normally uses a modal "sheet", not a modal dialog. A modal sheet does not block access to the rest of the application the way a modal dialog does. It also has a different look, "descending" from the main window's title bar as though pulled down ("unfurl" is Apple's term), rather than popping up like a dialog. A sheet is "document modal" rather than "application modal".
Given the above, I'm not sure it is a bug...