Not quite that simple. Google does at least partially own the platform and made a lot of noise about html being a better platform for being open before.
Everybody else just makes a business decision what they need to support. Native, Flash, HTML, browser variations.
For Google this is more of a power play, and a bit of a bait and switch as HTML was sold as being free of those power plays.
Google gives away all their client platforms as open source (i.e. Chrome and Android), so it's hardly a power play when compared with the other options.
True, Chromium and AOSP are open source. I think the point is that Google are playing both games here. They can quite happily compete with the Microsofts and Apples of the world when it comes to power plays, but they also want to see what comes from giving huge amounts of code to the developer community to play with. There are logical, long term business reasons for open-sourcing projects like Chromium, AOSP, Go, V8, WebM etc.