Let’s just say I know of someone intimately familiar with the issues involved from youtube’s side and what she told me about this issue as I understand it:
youtube does not delete videos with potentially copyrighted material for one simple reason. If they took proactive steps to remove, say, a Daily Show clip, but then failed to remove a copyrighted commercial from the 80s they would be liable under the DCMA for failure to pro actively remove other copyrighted content. Therefore, youtube instead takes the approach of removing no content unless the copyright owner asks (and they also provide tools to make this easier).