How exactly? There is no indication that Google wants to make VP9 patent encumbered, since their whole idea behind VP8 was to enable high quality open video codec for the Web and beyond. If VP9 will be their natural next step, it will be open as well. If Daala comes soon enough too - it will be another option.
http://www.webmproject.org/license/bitstream/
Why would Apple or Microsoft believe Google's patent and royalty free license given their behavior with Motorola's prior promises?
Because if VP9 is indeed similar to H.265 then you I would imagine a patent is being infringed somewhere. And since Google doesn't provide patent indemnification you can guarantee that some big royalties will be demanded from users.
Why would they design the codec to be vulnerable to patent attacks? VP8 was designed to work around threats from H.264. They surely applied the same logic for VP9 since they intend it for practical use and not as theoretical brain exercise. MPEG-LA spits threats all the time, including against Theora and VP8, but they have no teeth to bite.
Also, On2 which Google purchased holds lots of video compression patents, patents which codec's like h.264 and h.265 just as likely violates.
MPEG-LA does not offer any patent indemnification either.
