Google, Android and open source

Android, of course, is evolving, and this, of course, is a good thing for the mobile market.

But it seems like Google is an ardent supporter of open source software here?

Android Software Development Kit License Agreement
...
3. SDK License from Google
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3.3 SDK licenses, you can not modify, adapt, redistribute, decompile, or reverse engineer licenses. Do you need to use a new one? incorporating a part of the SDK.
...

... as they say, do not tru (c)
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Where then is the fundamental difference from the same .NET? Sense to change one closed proprietary platform to another?
Or am I still misunderstanding something?