Well, first off, Apple has an existing user base, part of which--believe it or not--hates mice with more than one button. Not throwing your existing users to the wolves as you strive to gain greater market share is a cause often championed, well, right here.

Second, Apple has had mice with right-click ability for years. There may not have been two, obvious buttons, but the shell rocked to left- and right-click. It was just the trackpads that, sadly, lacked the functionality for a while.

Finally, I can understand giving Apple grief over notebook trackpads that needed the assistance of a keyboard key to right-click, but USB mice for desktops? Sheesh. It's not as though the average (or even above average) PC comes with a mouse worth keeping. I'm happiest when I can drop the keyboard and mouse from a PC configuration, even if it only saves me five bucks. Any keyboard and mouse that comes with a PC is tossed, unopened, into the storage closet anyway.