because they've come from Windows where that is the behaviour. Much like using ctrl or cmd as a modifier - when you become accustomed to one, the other takes some getting used to.
I know a lot of people who are more familiar with Mac than Windows and still don't know what that button does and simply ignores it. Along with the other small oval button that's there in pre-Lion.
You would have a point there, except that the zoom button, as I believe it's called, behaves differently in different applications.
An example, in some browsers clicking zoom will zoom to the page width, however in iTunes clicking zoom shrinks you to the miniplayer.
The different behaviours are confusing and difficult to remember, and not only goes against user expectations but isn't user-configurable.
How is this an argument? If users expect a button to do a certain thing then that is what the button should do. Where the behavior was learned is irrelevant.
How is THAT an argument? Different users will obviously expect different things.
The green button in almost every case just resizes the window to the optimal size. Whereas the maximise button simply wastes most of your screen estate (which, bizarrely, many Windows users think is actually taking advantage of their large displays, whereas its really doing the opposite).
