Today I discovered a cafe where there's open, accessible free WiFi, not because the cafe is providing it, but because it's leaking in from someone who's working/living in the immediate area. So, ethics be damned, I sat there soaking up lots of lovely free WiFi for an hour or so. And I will of course do so again, next time I'm in the vicinity. However, I think if I had a computer it might be another matter.
I deal with these questions on a case by case basis.