This is entirely too complex and convoluted to be rational, but try this thought experiment on for size: How can Google break into the tablet market where Apple is already an undisputed leader?
Google sees that Apple and Microsoft are pushing for h.264 HTML5 support in the browser, but also sees that they can't muster a full 50% market share of the web between Internet Explorer and Safari. Firefox holds a plurality, with a bit of Opera which refuse to use the h.264. Chrome has become a wildcard. If Chrome supports h.264, Firefox will either lose market share and be replaced by standards compliant IE9 or Chrome or adopt h.264. Either way Flash video will probably decline, and eventually die, replaced by standard h.264 video on HTML5.
On the other hand, if Google rejects h.264, they know that Apple will be unlikely to support WebM from their competitor, nor will Microsoft. This pits the popular browsers (with >50% market share) against the corporate ideologs of Microsoft and Apple. This is a battle that could rage for years to come. Meanwhile, people around the world still want their Mean Kitty videos. Enter Adobe Flash.
Flash runs fine on Google android, and as a plugin to all desktop browsers including Safari, and is mature, well supported and well understood. Now, if companies have two options for broadcasting internet video; A mature, well understood and nearly universal Flash solution, or a volatile and codec muddled HTML5 solution. Which are they more likely to support? Sure, the big ones will support both, but the "long tail" small companies are likely to choose Flash. That would make their video unavailable to Apple iOS users, but given the choice of supporting 95% easily (everyone but iOS) or 45% (iOS, Safari and IE9) of web users, most of them are going to go with Flash.
So, because Google chooses not to support h.264, the HTML5 video wars continue to rage, and Flash continues to dominate Video streaming on the web. Apple, which doesn't support Flash and never will, has to keep encouraging companies to support h.264 HTML5 in order to keep sales of their devices high, but realistically that's a losing battle when h.264 HTML5 automatically excludes the majority of web users. So Apple loses, while Google's core business is unaffected, and some of their side business (Android Tablets) picks up.
As I said, this is pretty obviously a conspiracy theory, and is to convoluted and has too much reliance on Apple's bullheadedness to actually make sense, but I would guess that the scenarios outlined above crossed at least one Google exec's mind when deciding what Codec(s) to support in Chrome.
A far more likely scenario is that Apple and Microsoft will include a WebM codec in their browsers, and at worst this will set ubiquitous HTML5 video back a few months, as video chipsets implement WebM on chip, and ship those video cards.