If you've worked in customer service, then you're already deeply (and painfully) familiar with the adage "the customer is always right."
While there are plenty of kind and reasonable customers who have questions and concerns that are valid, there is also a growing subset of customers who shouldn't be allowed to speak to other human beings in public. These customers are grown adults who throw tantrums, call people names, and make the wildest threats over something as petty as a $4 coupon or a slice of pizza.
It's abundantly obvious that people who behave this way have deeper unresolved issues they are furling onto strangers, and eventually, even the most veteran customer service workers reach a breaking point.
While cool managers understand the importance of letting employees stand up for themselves against abuse and straight up bonkers requests, a lot of companies have stringent polices that punish workers for talking back at all.