[citation][nom]Jonnydough[/nom]A MAC is a PC.[/citation]
A Macintosh certainly is a personal computer. However, a Macintosh is not an IBM Personal Computer, containing a 4.77 MHz 8088, nor was it a PC-compatible computer. Until, of course, very recently.
Boot Camp lets you install Windows as a second operating system on an Intel Mac, although some problems were reported in the case of Windows 7, but I suppose you could still just install Windows directly on an Intel Mac if one wished to. Given that a PC running Linux is thought of as still being a PC, though, I suppose you could say that, hardware-wise, a Mac is a PC.
However, that doesn't change the fact that people will want to have a brief way to refer to an Intel x86 machine running the current version of Microsoft Windows. Those machines run certain software; a Macintosh runs different software.