I work in retail and over the years it's become VERY apparent to me that a certain percentage of the population feel that because you're providing a service/products they are paying for this somehow gives them the right to be rude, abusive and generally treat the person serving them in anyway they see fit.
At times I've been utterly disgusted at the behaviour of some members of the general public to staff. I've seen someone being rude and abusive all because they felt the tannoy music was a bit loud. On another occasion an elderly lady literally take a temper tantrum, stomping her feet and squealing like a 2 year old, because some products had been moved to a different part of the store.
I completely understand that people should have a right to complain if service is poor however those that act in this way usually tend to do so when either for very trivial issues or more often than not when the issue is completely out of control of the people they are abusing.
I normally find that pointing out to such people how would they feel if their mother/father/wife/husband/son/daughter were treated in the same way in their work gets them to stop and reflect a bit on their own appalling behaviour.