Just consider this (perfectly valid!) case: I, a German blog owner, blog about immigration issues and my blog post attracts a couple of US neonazis which use the Hitler salute, which is legal in the US, but forbidden in Germany.
As I am not in control of the comment server, I cannot delete the offending comment and protect myself from legal problems (it's a criminal offence after all).
Obviously the same problem arises for copyright violations - anyone remember the spread of the HDCP masterkey?