In legal regimes where you don't have vicarious liability for any activity short of activity that is authorised or abetted, having an open wifi could actually be a useful legal defence, compared to a situation where even though there is security on your wifi, you either get hacked or someone does something naughty but you're not sure who it was. You'd have a hard time proving this happened, and if you failed, the suspicion falls right back onto you; with open wifi, "ignorance is bliss"; the prosecution would probably have to downright prove it was your doing, because it would be hard for the court not to (rebuttably) presume it was an unknown user. At least in the UK, this is largely the case, though rightsholders have tried to force case law (Brown & ors v Polydor & ors) and legislation (Digital Economy Act 2010) in the direction of harsher vicarious liability.
Good point. So the discussed aspect depends on the country you're in. In Germany, the WiFi operator is held responsible for everything coming from his IP address. This resulted in some very ridiculous court cases in which 70 year old people are accused of downloading music and movies via BitTorrent.
