None of the signs I've seen in coffee shops say "customers only". They say "free wifi". That's good business. Whether everybody using it buys a coffee there today, or every hour on the hour, or once a week they get lots of people to come to their coffee shop and they sell lots of coffee.
If they start harassing their customers that in order to use the wifi access they *must* currently have a warm cup of coffee in front of them... is stupid and counterproductive.
In this story, even the coffee shop owner said that he didn't care that the guy was out in his car using the service. That's good business. Maybe the guy isn't a customer at the moment, but he may have been before or might be tomorrow. Plus... the sign just says "free wifi", not "wifi with a cup of coffee".
This forces me to ask the question: "What then were the police doing there?" Who complained if not the store manager? It seems like the police were there to nail THAT guy. What did this stakeout cost?