From the Viacom vs YouTube summary judgement (pdf):
The DMCA notification procedures place the burden of
policing copyright infringement—identifying the
potentially infringing material and adequately
documenting infringement—squarely on the owners of the
copyright. We decline to shift a substantial burden
from the copyright owner to the provider …
That makes sense, as the infringing works in suit may
be a small fraction of millions of works posted by others on the
service’s platform, whose provider cannot by inspection
determine whether the use has been licensed by the owner, or
whether its posting is a “fair use” of the material, or even
whether its copyright owner or licensee objects to its posting.
This is a deeply wise decision. Solomonic.
The next hurdle is a method to enable rights holders to police infringement at internet scale. They need to be able to send takedown notices fast enough (and have them take effect) for their will to be respected.