If anything this is a more clear-cut case of the intermediary bearing responsibility. Even if the design agency is responsible for originally stealing the work, they paid for the design without demanding proof that they actually had the rights to use the design, and they put it up on the internet without having proof that they had those rights. And this is all done to directly enrich themselves.

A far cry from 'common carriers' that, despite responding promptly to DMCA takedowns and other requests, still regularly get badgered and legally abused by IP rightsholders.