I'm sure they have written somewhere "Wireless internet access is provided free to our guests."
It's up to the shop to decide who a guest is. I'm going to guess a guy taking advantage without paying is not.
If it's free; how does one steal it? A guest is anyone who visits.
However, if the sign said "Complementry Wireless Internet For Our Patrons"; then that would be a different story. However, if they simply said "Free Internet", then free = free.