> Isn't it the case that a calling a method with (say) null parameters where they are supposed to be non-null should raise an exception?

Not really. Checking parameters in every method (especially an OO system) is a sure way to have poor performance. Null values as other kinds of testing should be done in the areas of the system that create and receive objects from outside.

Normal methods should just work find even if a parameter is NULL. Although this doesn't look nice, it is a way to make the system testable, in the absence of true NullObjects.