Imperfect analogies are imperfect.
Using someone else's open WiFi does have an impact on them. Though it's likely minimal, there are situations where you could definitely impact them - using significant bandwidth, downloading things that get them naughty MAFIAA letters, or other issues degrading their network. It's not completely zero impact like watching a TV though an open window, but it's not fully utilizing a resource and taking it away from it's owner.
However, it IS unauthorized access. The various 'digital tresspass' laws (most of which are a crock of ... erm stew) make this illegal. The owner may be less than tech savvy and *should* secure their network ... but anyone not given permission to use it, definitely don't belong on it.
Open NYC WiFi ... I'm going to guess that buried in the terms is (or eventually will be) some clause about not retransmitting, selling, porting, routing, etc. the wifi. Or maybe not, but if they put these near housing too many people will use it and they'll become useless.