They could lock it if they want to
This would obviously be bad business for them. This isn’t a private house, it’s a public place where they offer Wifi for their customers. If they locked it, it would defeat the whole purpose. They would have to give the password to all their customers which would just be a pain, and you could get it too anyway so what would they gain other than someone having to come in in order to use it the first time. I think that their business model is that they assume that if they offer Wifi, many people will come into the store to use it and might buy from them. People in the parking lot might also buy from them. The fact that you don’t plan on buying is the issue. If there is a chance that in all the times that you use it, one time you might come in and buy a coke, then it is worth it for them. It might even be worth it for them if you refer them a customer by telling a freind that you use their parking lot. Even though your friend might be Jewish, he still might buy a coke. Or he might refer a friend…
In terms of using their bandwidth, it is a public place where anyone can, and is encouraged, to log on so I assume they have somewhat good service to handle all this bandwidth. If not, then their customers would be experiencing slow connection even without you. I doubt that one extra person on the network makes that big of a difference. Feel free to argue this last point.