Despite the "PC vs. Mac" ads, PC still stands for Personal Computer, and it is used to differentiate between machines that run full operating systems and have significant resources vs. microcontrollers that don't. From Wikipedia: "Personal computer, a computer whose original sales price, size, and capabilities make it useful for individuals. Commonly running either Windows, OS X or Linux." The important thing here is that while you may be able to host certain simple USB devices with a microcontroller, it's much more difficult to host complex or arbitrary devices given the limited resources of a microcontroller.
And you're right that I didn't state the keyboard/mouse example correctly (sorry, we're busy here!), I'll edit it as soon as I get to a PC.