That’s an implementation detail that you shouldn’t care about. The important thing is that now that the Web API uses the DependencyResolver to resolve dependencies for many different facilities, you will be able to use a real dependency injection whenever you want to plug into those facilities. So in your code you will be using a real dependency injection. If Microsoft didn’t use the DependencyResolver then it would have been you that must have used it (as a service locator anti-pattern) in your code in order to resolve dependencies when you want to implement some custom functionality. This would have been bad for you. Now it’s bad for Microsoft but you don’t care about them.
Thus I’m left curious and confused why Microsoft would use a service locator in 2012.
Because designing a framework is not the same as designing an application using a framework. There are some different things to take into consideration when designing a reusable framework such as ASP.NET MVC rather than just what’s written in the books. Some example is to design the framework in such a way that a person using this framework will be able to take advantage of the best practices written in the books in his code using this framework.