IANAL. Germany does have a common carrier status for ISPs just like the US. It doesn't extend to private citizens.

The liability in cases like this is not really settled, though it's not looking very promising. It's already very difficult to find open Wifi, which is at least partly due to concerns about liability and their widespread discussion in the media. OTOH I think there've been Tor exit nodes in Germany for a long time, though not without subjecting their operators to some legal harassment (and bills).