They don't have to indemnify VP8 adopters. VP8 is only patent-free if you agree not to do what MPEG is doing. And while it may be true that MPEG holds some patents that apply to VP8, Google clearly holds some patents that apply to H.264 (because this is a huge mess.)
So they can sue, but it will be an ugly battle, and unlikely to finish before the H.264 patents expire. It's really not a good idea.