Legal liabilities:

Suppose the home network user has no security enabled and the network connection is always available. A spammer could drive by in the middle of the night, connect to the unsecured internet connection and then send out thousands of e-mail spam. The home's ISP now bans the user and shuts down his or her account for sending out the spam. Now lots of explaining.

Another issue that may not come as often but has a much worse consequence is when the home user can connect to his or her workplace using a VPN connection (virtual private network) and does not have security enabled on the their home network. Assume the home network user is a vice president at a bank or maybe a doctor at a hospital. He or she could use their VPN connection to make a secure connection to their workplace's network. Assume the user's neighbor looks for an easy internet connection and connects to this unsecured network. All is fine as long as neighbor just visits the internet. But suppose the neighbor is just a little curious and browses the local network and sees unfamiliar drives. A little more browsing and this neighbor also has a VPN connection to the hospital's or bank's secure records. And then just for fun, he or she probes to see what is available. The next day, the doctor or VP is called into the Director of Network Security and then must explain why he or she was viewing files that they had no need or authority to see. There can be severe consequences for the doctor or bank employee for such unauthorized exploring.

Here are the solutions.