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Site owners used to be held liable for even unintentional infringement, even when they police the site and they remove infringing works immediately. In response to this, the Congress passed the Digital Millennium Copyright Act (DMCA). Under the DMCA:
[a] service provider shall not be liable . . . for infringement of copyright by reason of the storage at the direction of a user of material that resides on a system or network controlled or operated by or for the service provider, if the service provider
(i) does not have actual knowledge that the material or an activity using the material on the system or network is infringing;
(ii) in the absence of such actual knowledge, is not aware of facts or circumstances from which infringing activity is apparent; or
(iii) upon obtaining such knowledge or awareness, acts expeditiously to remove, or disable access to, the material;
(B) does not receive a financial benefit directly attributable to the infringing activity, in a case in which the service provider has the right and ability to control such activity; and
(C) upon notification of claimed infringement as described in paragraph (3), responds expeditiously to remove, or disable access to, the material that is claimed to be infringing or to be the subject of infringing activity.
So, the DMCA protects site owners who follow those guidelines and have a policy
for reporting and removing violating material. Furthermore, site owners escape liability by disclaimers and having users agree that anything they are posting on the site they have legal permission to post or are the owner of the material being posted.
See also: http://www.dmlp.org/legal-guide/protecting-yourself-against-copyright-claims-based-user-content
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