Because it's ironic that he's being a stickler for standards when his own site is broken.
Edit: indeed, this is a bug: there are different CSS styles for devices with < 480px screen width and > 480px (via @media CSS rules). Desktop browsers pick the latter style and ignore "viewport" meta tag. iPhone picks the former style and takes into account viewport. iPad picks the desktop style and takes into account viewport, which disables zoom (intended for the iPhone style version).
Now that I debugged the problem, can we instead report bugs to website designers instead of commenting here on HN?