The problem the previous poster was descrbibing wasn ot when you act as an ISP, but when someone manages to log into your wifi (either because it's unprotected or unintentionally vulnerable), and performs criminal activity. The US legal system has reliably shown itself willing to pursue the owner of an IP address, without going to the trouble of showing the owner was in fact the one using the IP for that purpose.
Someone using your wifi means you are Providing Internet Service to them. Is the problem that you're not incorporated? If it's based on IP allocation, what if your home connection has 8 IPs and you give one to each user?
If your WiFi get's hacked because it had too low security standards, like WEP, then you're still responsible for what's being done with your line. But you're irresponsible if you have taken care of security by following the router's wizard. There is no such case, that the manufacturer was blamed to offer too low security until now though (afaik).
What metric does Germany use to distinguish between 'ISP' and 'not ISP' when it comes to providing internet service?