But what you described isn't a "problem" on it's own. Subcontracting is a thing. It's not a bad thing on its own.

You're describing the "what" - you're not describing the "why" is that an issue...

Is "Joe's delivery company" not already required to have liability insurance? Do we think this insurance is not big enough? Should we make it bigger for all delivery companies or do we need to single out Amazon's liability insurance? Do we think that a delivery company delivering the same package for Amazon vs. FedEx vs. Mary's Flowers Inc. vs. NYC Movers Inc. should have a different amount of liability? (e.g., if that delivery driver injures a pedestrian, should we require that NYC Movers Inc. covers fewer costs than Amazon)?