Sub-contracting and outsourcing can be used to distance companies from the people who do work for them in a way that protects bad practice and exploitation. That's a mahor reason why it has probably increased in recent years as a way for exployers to avoid regulation that is meant to protect workers and quality. It's a way of avoiding responsibility. And as the court case Teribus quotes demonstrates, it is a pretty successful way of doing that.

It happens in all kinds of areas - local councils do it. When you complain about some faulty bit of work repairing council property the council says it's down to the contractor, the contractor says it's the subcontractor, the subcontractor says it's the council...

It's a serious problem, and too often gets ignored. But most of the time those engaged in this, at the top especially, are natives.