Digital Millennium Copyright Act
The DMCA criminalizes production and dissemination of technology, devices, or services that are used to circumvent measures that control access to copyrighted works (commonly known as DRM) and criminalizes the act of circumventing an access control, even when there is no infringement of copyright itself. It also heightens the penalties for copyright infringement on the Internet.(Wiki)
This act has made things very easy for copyright owners when it comes to keeping copyrighted works out of the public sphere. In fact, when a takedown notice is received it is in the best interest of the recipient to simply do it because if the potentially infringing work is removed then they are no longer liable. Of course this means that even if works are only POTENTIALLY infringing, then someone might receive a takedown notice and to not comply could mean being sued by the RIAA, MPAA, or an individual corporation. Such was the case with YouTube when Viacom sued them to have all copyrighted works taken down. Anything with even the slightest reference to Viacom owned materials was taken down.