Its by design. An interface is the point of communication between one program entity and any other program entity. If you restrict access to an interface, its no longer really an interface, if you follow?
Why would we want to declare an interface as protected? If you declare an interface as protected then only the other classes in the same package (or subclasses of the interface) could access it. With an interface the whole point is to allow ANY class to implement it so that the implementing class is guaranteed to have the exact method signature as any other class that is implementing the interface. They allow us to to get around the problems associated with multiple inheritance, but stil have that method signature guarantee. this way we know how to call the method and what to expect in return.
Besides, what�s to protect in an interface? None of the methods do anything until the interface is implemented and they are themselves implemented.