IANAL but it seems they could lose safe harbor protections provided by DMCA and be liable for ALL copyrighted material going over their system:
"Section 512(a) protects service providers who are passive conduits from liability for copyright infringement, even if infringing traffic passes through their networks. In other words, provided the infringing material is being transmitted at the request of a third party to a designated recipient, is handled by an automated process without human intervention, is NOT MODIFIED IN ANY WAY, and is only temporarily stored on the system, the service provider is not liable for the transmission."
"Section 512(a) protects service providers who are passive conduits from liability for copyright infringement, even if infringing traffic passes through their networks. In other words, provided the infringing material is being transmitted at the request of a third party to a designated recipient, is handled by an automated process without human intervention, is NOT MODIFIED IN ANY WAY, and is only temporarily stored on the system, the service provider is not liable for the transmission."
http://en.wikipedia.org/wiki/Online_Copyright_Infringement_L...