They have a local monopoly on their hardware. It's like cable companies: there may be several of them in a country but if there's only one in each city, they have a local monopoly that they can abuse.
My third-party iOS browser uses its own renderer and they don't seem to have issues with it. However, it is designed for niche cases and does not implement the full W3C spec., so I guess that is what they really have issues with?