Why is TGB forced to 16bpp in fullscreen mode?

bool OpenGLDevice::setScreenMode( U32 width, U32 height, U32 bpp, bool fullScreen, bool forceIt, bool repaint )
{
...
   if ( newFullScreen )
   {
      if (newRes.bpp != 16 && mFullScreenOnly)
         newRes.bpp = 16;