my guess is there are certain relationships in which a duty of care between A and B that cannot be delegated away to an independent contractor.
i'm making this up here, so be careful. let's say a hospital corporation owes a duty of care to its patients. it uses independent contractors to do some procedures on patients or something. i would think that the independent contractor exception wouldn't work there. maybe a school to student relationship would work.
whatever the relationship might be, i think certain folks owe a duty of care to some other folks that cannot be delegated away through the IC exception.
That is exactly the basic idea. Generally parties cannot be held liable for the actions of their independent contractors. This is what differentiates the employer/employee (respondeat superior) relationship from the primary/independent contractor relationship.
That being said, there are some activities so dangerous or so whatever that the primary cannot delegate it to their independent contractors and escape liability.