I've noticed the same. And in a majority of the cases, the videos play just fine on an iPad, which is pulling an H.264 version from somewhere.

The FAQ page the video links to mentions the following:
Quote:
Videos with ads are not supported (they will play in the Flash player)
On Firefox and Opera, only videos with WebM transcodes will play in HTML5
If you've opted in to other testtube experiments, you may not get the HTML5 player (Feather is supported, though)


I'm guessing it's some enforcement of ads, though if a much smaller shop like The Verge can do HTML 5 ads, I have to wonder why Google can't.