I would venture a guess that this is to make more accessible the Android platform to existing ActionScript developers. I can't imagine many Android devs would pay thousands to buy and learn Adobe tools when they can build the same software with the tools they know.
Or not pay thousands and just compile for free using a handful of OS tools that work just as good, perhaps even better. (ie: http://www.adobe.com/products/flex/)
I don't think Adobe's Builder [tool] is free?
Flex SDK is open source. You can find it on that page.
No the flex builder isn't free. But the flex sdk is opensource