The best reason for not shipping a dual+ button mouse for the Mac is that software makers could well start to depend on you having one and not following the UI guidelines as they should. That’s fine, and you can plug a new mouse into your computer, but you’re still paying Apple for their crappy single-button one which sits in your drawer and while Apple’s software can obviously recognize multiple buttons on mice it only lets you set them for Expose and Dashboard. There’s no reason why you shouldn’t be able to choose what you want at checkout, and have full support for standard mice with multiple buttons and scroll wheels, as developers still couldn’t depend on you having one.