Oh, I agree they can kick you out. But getting a coffee and walking away is very different from using the WiFi and walking away.

Not really, you're using a service, the cost of that service may be incidental but the classic superman/office space bank hack of diverting all the partial cents on interest to a separate account isn't legal just because each action is so small... It is different because it'd probably be silly to enforce that everyone using their wifi has paid for a drink, but there isn't a distinction there ethically or legally... Unless you think that wifi should be a free social benefit for everyone everywhere - but that particular wifi is being provided by starbucks for a cost to them.

I just think it would be very hard to legally argue that there is any implied contract that you must buy a coffee to use the WiFi. But then again, IANAL.

There is a literal contract for Sbux wifi. You need to agree to a ToS when you join the wifi network. That ToS includes "you must be a starbucks customer".

Obviously what a "customer" entails is up for legal debate, but it generally does not include people who purchase nothing from Sbux.