Mail will open up a viewer window if you have no windows open. The thing is that programs only open the window if you switch to them by clicking their Dock icon or double-clicking them in the Finder, not if you command-tab. Basically, if you do something that would open the program, the program will pretend like it's being reopened and do whatever it normally would when you open it. You can't open a program with command-tab, so you don't get the behavior in that case.