The one button thing is more about consistency than a limitation.
If you force app designers to work with one mouse button as being the lowest common denominator, you ensure that all apps are designed to be accessible with only one mouse button. That way you don't need a contextually sensitive right click to enable functionality like you do in some poorly designed Windows apps (and probably Linux ones too.) This fits in well with OS X's overall menu structuring too.
If you look at it in Windows terms, think of the Control Panel. You can easily create apps on Windows for configuration functions that don't have to be installed in the Control Panel. It makes sense to users that they go there, but there's nothing forcing them to use it. If expecting a consistent user experience makes me a retard, retard me up, I guess.
I don't know of any Macs being shipped these days that don't actually support more than one button on the shipped hardware. All the Macbooks and Macbook Pros can enable a right button click through the trackpad by having more than one finger on it (like for scrolling) and clicking with the button. All the other Macs ship with a Mighty Mouse that can be configured for more than one button.
