Following my post on Ghetto Wifi, I passed a pub yesterday with a sign "Free wifi for customers only".
Thought of another perspective on this problem. How long do you remain a customer after you have bought a drink? One hour? One day? Is there some kind of dwindling right to use the facilities as time passes? If you bought a coffee within the last half hour then you are a full-status customer with full rights; if you haven't bought a coffee in the last two hours, then maybe your customer rights are weaker.
Let's look at some more examples. If you have lunch in a restaurant, and then do some shopping, can you then go back to the restaurant to use the toilet before driving home? (If you left a reasonable tip, then maybe the aura of "customerhood" still lingers.) If you have lunch in a pub, can you leave your car in the "customers only" car park for the rest of the day? Some establishments have strict limits - for example, some supermarkets have free parking for two hours, and then start clamping or ticketing people who stay longer.
Obviously there will always be some people to try to cheat - to use the facilities without buying anything at all. The point here isn't whether this is possible, or the extent to which the establishment tries to make you feel welcome (in order to convert you to future customer) or unwelcome (to reserve its facilities for genuine customers), but what moral rights you have in the first place.
Some people might think that the "customers only" goes without saying - you shouldn't need a notice - but presumably there are some people whose behaviour will be influenced by a reminder that these facilities are intended for genuine customers.
So it comes down to a question of what counts as a genuine customer, and for how long.