Is one who leaves their wireless internet open for others to use sinful if someone uses it for illicit purposes?

If you do not know or do not predominantly believe that the wireless network will be used for unlawful purposes, then there is no harm in making it available to everyone. However, if you predominantly believe it will be used for disobedience, then it is not permissible to assist in that, due to His Almighty's saying: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression."