Monitoring their content with a human-in-the-loop would take the outside of the conditions for DMCA safe harbor: http://www.chillingeffects.org/dmca512/faq.cgi#QID129

This is a key condition for all service providers under DMCA -- if you have someone that is looking at the content, you are then liable for all the content on your network.

In essence, this $6/hr intern would cost justin.tv their ability to defend against suits via the DMCA. There would simply be no way for them to operate as a UGC business.