I have some reservations about it, but not enough to refuse to use it in an emergency. I already have a net connection at the places I frequent.
I would assume a coffee shop owner invites the use. I do not expect that the average Joe Neighbourhood who leaves his wifi open invites access, even though he may be ignorant and not realize the consequences.
However, the computers don't distinguish between the former and the latter's intended use. The computer just sees an open acess point.
I once accidentally used an open wifi for weeks without realizing it: my machine just connected to whatever it could find open. Whoops! Once I realized it I set it to use my own closed wifi.
In my opinion, while technically it is an open invitation, it is impolite.
Even if you are dangerously ignorant and naive about security of any sort, it does not mean one should have bad things happen to them by opportunists and criminals, nor does it mean criminals should not be punished. However my righteous contempt of criminals does absolutely nothing to stop this sort of thing: better security practices, on the other hand, do.
Clear as mud? :)