Why does OS X do this? ...

I'm new to OS X so just bear with me. This is something different from Windows ... eh, actually it doesn't even apply to Windows.

When I open Safari, the OS X menu changes to Safari. When I close Safari, the menu is still Safari. I would think the menu would change to Finder.

If I have iTunes and Safari open and I'm using Safari, the menu is Safari. If I close Safari, the menu is still Safari. I would think that the menu would change focus to iTunes and change the menu to iTunes but it doesn't until click on iTunes.

It appears that closing an application doesn't change the menu to the current active application. It's not a big deal, I just wonder what the reasoning is behind this UI decision.

Unlike Windows when you close the window, your just closing the window. You have to tell the program to quit either by command -Q or from the Finder bar. Also you can close it by right clicking (or command and click on the single button mice) it in the dock and telling it to quit. You can also quickly switch between programs by using Alt and Tab

Edit: sorry guys

Because when you close an application's window you are not quitting the application, it is still running.

Contrary to Windoze, applications on the Mac are not window-centric i.e. you can have the app running with more than one window open and you can have the app running with no windows open as well: the app is ready waiting in the background for another window to open at your convenience and it is not using up system resources if it is not used.

To quit the app, you need to choose Quit or do Command + Q.

Edited to add:

mac0s9user beat me to it.

Closing the application does not Quit the Application. You are just closing the window but Safari for example is still running. Either Right (Control) click the Safari icon in the Dock and on Quit or on Safari in the toolbar and on Quit or Click Command+Q. All will quit Safari completely and it will no longer be running or part of the Toolbar up top.

Edit: 2 people beat me to it! You guys are fast!

My fingers are still smokin'...

This is different from Windoze but to me it is a good feature. I like controlling the app I am in at that time.

FWIW, working with Windoze 2000 drove me nuts — OK, more nuts. I'd close a window and the app would quit. It's a good thing the office had a foam-rubber hammer.

Until I upgraded my RAM, this ruined my system performance xD, As a mac newbie 18 months ago I couldn't work out why it was so slow, then I looked at the dock and realised I had about 30 programs running at once xD, My usual array of constantly running apps (aside from finder and dashboard) are safari, mail, iTunes and Last.FM

It is quite nice to already have an app running to speed up startup times ^^

This is one of those things that is just "different" between Windows and Mac. There is no "right or wrong", each has it's own way of doing things.

Activity Monitor can be useful to see what amount of RAM, CPU etc a specific program is taking.