I walked into a McDonald’s to use the restroom and that’s it. There’s a sign posted that says restrooms are for customers only.

I consider myself a customer as I eat at McDonald’s at least twice a month. I just didn’t purchase anything at this particular time cos I just needed a restroom break.

My question is Is this wrong? Are you only a customer when you buy something at the time you enter the store? How often have you walked into a store at the mall and left without buying something? Are you still a customer then? What’s the rule here?