Actually in NY it's quite legal ( except in westchester ) . In other states any decent defense lawyer could get the case thrown out because very few states specifically define what "unauthorized access" means if people decided to fight, but most pled to a simple fine if they were convicted at all.

A few enlightened states are even moving to push the burden to the WiFi operator such as California and New Hampshire, with more states following suit. Due to proliferation of WiFi devices more states will have to deal with the issue of open networks. They can choose to criminalize what is the default behavior for most Wifi devices or they can choose to blame the AP operator when they, ignorant or not, leave access open.