Passengers' contracts are with the airline, that's who they pay their money to. If their suppliers/subcontractors get things wrong then the passenger deals with the airline* and it's up to them to sort things out operationally and financially with their service providers. The same is true about any other aspect of the service, lost bags, etc, it's irrelevant to the passenger "who did it", their remedy route is through their airline.
* : Except Ryanair, who tell every complainant to f-off and so it's not worth the effort.