Part of the reason that some librarians prefer to call patrons "customers" is that they believe in ideas such as "the customer is always right".  But here's a good list of Five Reasons Why “The Customer is Always Right” is Wrong.

We run more than 3 million people through our books every month. One or two of those people are going to be unreasonable, demanding jerks. When it’s a choice between supporting your employees, who work with you every day and make your product what it is, or some irate jerk who demands a free ticket to Paris because you ran out of peanuts, whose side are you going to be on?