Question: "What problem does mere conduit cause?"  Context: "The U.S. Digital Millennium Copyright Act (1998) and the European E-Commerce Directive (2000) provide online intermediaries with limited statutory immunity from liability for copyright infringement. Online intermediaries hosting content that infringes copyright are not liable, so long as they do not know about it and take actions once the infringing content is brought to their attention. In U.S. law this is characterized as "safe harbor" provisions. Under European law, the governing principles for Internet Service Providers are "mere conduit", meaning that they are neutral 'pipes' with no knowledge of what they are carrying; and 'no obligation to monitor' meaning that they cannot be given a general mandate by governments to monitor content. These two principles are a barrier for certain forms of online copyright enforcement and they were the reason behind an attempt to amend the European Telecoms Package in 2009 to support new measures against copyright infringement."  Answer:
barrier for certain forms of online copyright enforcement