Here's the scenario: You have an open WiFi (i.e. no password required), someone (maybe you, maybe not), uses that IP address to download a copyright work, someone (probably a copyright troll) sends a subpoena to your internet service provider and finds that this happened, you receive a letter from a copyright troll attorney that says in basic terms: "You are a horrible person. A copyright protected work was illegally downloaded using your IP address. It was entitled something that included "hot", "wet" and a bodily orifice in the title. You should be ashamed and if you pay me $3,000 now, it will all go away and your wife/girlfriend/scout troop/sunday school class will never know. Otherwise, we can sue you for negligence because your WiFi was not protected and we don't even have to prove you did the download."
Maybe this comes as a huge surprise to you, maybe it doesn't. However, will the negligence claim fly and allow the trolls to tag you with liability even if they can't prove you actually did it? A couple of courts have said no. Last week the U.S. District Court for Northern California in a case styled AF Holdings LLC v. John Doe and Josh Hatfield held that the mere inaction of not protecting your WiFi was not negligence because the defendant did not owe a duty to the plaintiff to take an affirmative action to protect the plaintiff's intellectual property. In addition, the court held that this was still a copyright case and state law of negligence was preempted by the federal copyright statute. And to further make a point, the court found immunity for the defendant under Section 230 of the Communications Decency Act.
So, it seems to be the trending opinion that you aren't strictly liable for contributory infringement for just leaving your WiFi open. Seems right to me.