A person walks into a pharmacy and buys a soda. That person is a customer. If that person goes to the pharmacy counter, he is a patient to those behind that counter. What's the difference (from a Star Trib article about retail medical clinics)?
"Patient is a passive term," he said. "Customer is an active term. We never forget that the customer is paying the bill."
I'd also argue that the term "patient" represents a paternalistic mindset held by many healthcare providers, including pharmacists. Therein is one reason why many medical practictioners tend towards paternalism when it comes to health care provision and tend away from allowing people to have more choices in who they receive their care from (such as the case of worrying over the care customers/patients receive at retail medical clinics).