Why the Mac OS defaults to one mouse button (though it has supported and made use of two button mice for over 15 years now):
1. Programmers on Mac OS must assume that the user has only one button and may not have two, so this forces them to put commands in the menu bar at the top of the screen - not bury them ten levels deep in context menus and make them otherwise inaccessable.
2. Ever try to teach grandma to use a computer? Or work tech support for newbies? “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?” “Left click or right click?”
Yeah, it gets really annoying for user and instructor. Having one default button gets around that and reduces support costs.
The actual guidelne states "Always ensure that contextual menu items are also available as menu commands. A contextual menu is hidden by default and a user might not know it exists, so it should never be the only way to access a command. In particular, you should not use a contextual menu as the only way to access an advanced or power-user feature."
Users of an older single-button mouse can simulate a right-click by holding down the 'control' key on the keyboard while clicking the mouse.
Apple's design guidelines for contextual menus are available at this link - Apple Human Interface Guidelines
I have to use a PC at work. Since I have always owned an Apple/Mac, the ‘right click’ has never been an issue. I learned that was the only way to enter certain ‘new’ info on the system at work.