I think it's a combination of factors. In the first place, a good number of people who are not in the service industry look down on those who are simply because they can. Gives them an opportunity to feel superior to someone. I learned that lesson hard and fast while putting myself through college working retail.
That combined with the fact that modern life is stressful as all hell, and with the fact that some people are just flippin' assholes, helps to explain her behavior.
In a situation like this, unless the pizza is swimming in sauce, I'll eat it. If I don't care for it, I'll mention that to the waitress. If they don't do something to make me happy, I'll firebomb the restaurant
In reality, I'll pay, and then never come back.