Worse than that, you have probably (as I have) reported plainly-obvious security vulnerabilities with at least a faint twitch of nervousness that the recipient is going to flip out and start suing or something. Worse than no expectation of reward from a utility point of view!

If you use someone's non-protected wifi for illegal activities, like downloading porn, then the owner of that "hotsport" is responsible at court. (At least in Germany afaik)

This is no different and I'm sure that you could win at court with a good lawyer

In the US when you provide wifi to people you are just as protected as your ISP. You might be in violation of your ToS but that's an unrelated issue.

What metric does Germany use to distinguish between 'ISP' and 'not ISP' when it comes to providing internet service?

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).