As I infered to before, it is poor business sense to charge zero for your services. I think the lifespan of any company with this slogan would be colourful, but alas, short.
Lets face it. There is 'No free lunch'. Someone is paying for ALL Wifi services. Now if they then offer this as an incentive to attract you as a customer for other services, the cost will be factored in to what you are paying for those other products. Simple economics.
The closest we get to FREE is the local library scenario. But at a closer look we see this is paid for by local councils. Local councils are for the use of local residents. Local residents pay local council tax. It is the payment of this local tax that gives the user authorisation to access the library WiFi. It is not in fact free. Should you take your Laptop/PDA to the local library, you are in fact completely justified. Logging onto an unsecured network 3 doors down is not.
Now back to the original argument. I believe if you log onto a network without authorisation, it is theft of bandwidth. I wouldn't get sucked into the illusion that the owner of the connection is in the wrong. Not securing wireless networks is not, at this stage, a criminal nor civil offence.