bool operator ++ and --

Today while writing some Visual C++ code I have come across something which has surprised me. It seems C++ supports ++ (increment) for bool, but not -- (decrement). It this just a random decision, or there is some reason behind this?

This compiles:

static HMODULE hMod = NULL;
static bool once = false;
if (!once++)
    hMod = LoadLibrary("xxx");

This does not:

static HMODULE hMod = NULL;
static bool once = true;
if (once--)
    hMod = LoadLibrary("xxx");