This is completely different, YouTube is a free service, Google has no obligations to third parties. AFAIK Amazon Web Services are not free, if Amazon imposed this sort of restrictions on their customers, I am sure they would show Amazon the door.

Amazon is actually relevant here, because their Kindle Fire browser is partly cloud based. Microsoft may say that HTML5 on the Win Phone isn't feasible, but they own a giant cloud based server farm (Azure) just like Amazon. So if Amazon can make a browser that splits tasks between the cloud and the physical device why can't Microsoft handle HTML5 this way?

.... and be in breach of their own contract with those sites, who pay amazon for the privilege - for which they would lose in court if sued.

If microsoft had a signed contract with google, (which would probably entail payment to google) they would have a case. As it stands, they don't.