A view Class to mediator Class mapping means that there may be multiple mediator instances for a given view type. How would RL know which instance to inject?

mediatorMap.mapView(SomeView, SomeMediator);
..
addChild(new SomeView());
addChild(new SomeView());
addChild(new SomeView());
..
[Inject]
public var view:SomeMediator; // which one of the 3 mediator instances should we inject? it makes no sense.

If you want to inject a mediator, and you know there will only be one instance, you need to map it yourself. You can do that in the mediator's onRegister hook:

function onRegister():void {
  injector.mapValue(SomeMediator, this);
}