Apple branded mice have had right click functionality since 2005 [1]. However, only a minority of OS X users uses a mouse, and an even smaller group uses contextual menus on a regular basis.
Everyone is missing the point, though. Having 1-button as the default (as it is in OSX) forces developers to make better UI decisions. Plain and simple.
When you can hide the complicated stuff in complex right-click actions and middle-click sub-actions and various other hidden UIs, people tend to do it, because it's easy and doesn't require too much thought.
When you have to assume that the user has only the "Point" and "Click" actions, and not an ever-deepening rabbit hole of right-click, middle-click, side-click, etc. etc. then you have to think a little harder about how you design your interface. You might add things as top-level buttons, or make a more useful interface on the primary click.
You might say this leads to too much variation in the UI space, since there isn't a standard way to deal with operational complexity, but I highly disagree. It leads to UIs which are better thought out, where more of the action has to be exposed and discoverable rather than hidden. It forces developers to make some difficult decisions about the complexity itself, too, which generally leads to simpler if not just better-realized apps. Even just the fact that you have to think a little harder about your UI is enough; a second-order effect that is clearly visible in the end result and the reputation of OSX and its apps.
All of this because Steve Jobs didn't let them tack on the extra buttons. Because he kept the primary interface simple. Because he knew it wasn't just about clicking, but about the whole ecosystem of software for which the mouse API was core. It was not just a mouse, but a genesis of software UI decisions.
Think like that, and you start to understand the single button and why it's actually important. Then you still go into settings and enable right-click... because if you're reading this, you are not like the majority of users (I mean, seriously, that's day one of HCI 101).
By this logic, why don't we all use one-button keyboards?
On top of that, most phones now have excellent voice input, so you can skip the keyboard altogether.
All of this because we made devices without keyboards at all. Necessity is the mother of invention.
http://venturebeat.com/2009/01/26/notebooks-now-make-up-over...
Of course, some of those notebook owners will also use a mouse. And I'm speaking as a Mac Mini user with an Apple branded mouse, but I guess your original point probably stands.
I wish I didn't have such a reliance on the mouse, but since transitioning from PC to Mac, one of the few persistent problems has been keyboard shortcut support. I can't tell if I've failed to learn sufficiently, or if it's just lacking, but certain applications - e.g. Finder - just seem to be impossible to navigate without a pointing device.
Here are some other solutions: http://superuser.com/questions/90378/
Here you go: http://i.imgur.com/uYDokqn.png
I find I use these (Alfred now, formerly Quicksilver) to open the right finder window first time, eliminating the need for most uses of the controls on the windows.
(also, once you install one of these you'll never need to open the /Applications directory again.)
I'm not saying that everyone does the same, but if you want to prove your assertion that most Mac users don't use mice, you'll need something more compelling than the fact that they don't ship with one.
For mouse use on the Mac to be the most used input method, over one third of Mac notebook users would have to permanently use a mouse instead of the built-in trackpad. You can go to any Starbucks, airport, or school to see that that is not the case.
[1] http://www.macworld.com/article/1167850/apple_revenue_profit...
Did you happen to hear the term "selection bias" before?
As I've said, I'm not saying that most Mac users do regularly use a mouse (although every single Mac user I've ever talked to about it at work regularly uses one). But to demonstrate whether it's true or not, laptop sales figures and anecdotal evidence based on people's habits at coffee shops isn't particularly compelling.
I for one, use it everyday to open links in a new tab. Cmd+click doesn't work on every website you know.
(I have no proof for any of this. I have watched all kinds of people use computers for 25 years, in all kinds of settings.)
I've worked a lot with FCP (and other editing suites) and PS, and everyone I see, especially pros, seems to make heavy use of keyboard shortcuts and modifiers, rather than fishing around in menus.