A coffee shop in Washington state had a guy arrested for using their "free" WiFi signal for three months without buying anything. The alleged WiFi poacher apparently just sat in his car in the parking lot nearly every day and used the Internet access without buying anything.
The shop finally called the cops, who told the guy to leave. When he returned later, the cops were called again and this time, they charged him with theft of services. The law here is murky, but it is a good example of how poorly many WiFi hotspots are managed. With different software or hardware, the store owner could limit the amount of time any one person uses the service per day, or could simply block the MAC address (a hard coded network address in each computer) of that computer.
Thinking about setting up a hot spot? Get good advice before jumping in.