In the U.S., it is well known that wait staff are paid little, so to
some it seems as if customers are guilt tripped into tipping whether they
feel it's earned or not. To some extent this is true, and as noted it
has come to be considered part of the cost of the meal. That however is
no excuse for poor service. A server who is unprofessional or rude or
simply does not do a good job, is not deserving of a generous tip.
Some people leave a small tip anyway, knowing that the person still has to
eat, but that tip will be directly in line with the quality –or lack
thereof- of the service.
This goes back to the discussion of how much you are willing to pay for
a meal. You cannot reduce the menu price, but you can certainly lower
the tip from 20% to 10% if the meal is not as enjoyable as it could
have been. You decided to pay 10% less for that meal, because the
experience was not what you expected. Some people enjoy having this control
over the situation and wish that every industry offered such a mechanism
for insuring good service.
On the other hand, there are customers that take advantage of the wait
staff's reliance on their generosity. No matter how excellent the
service is, they do not leave a decent tip and simply use any excuse to
leave less than the customary amount. This is just their way of saving
money. It is of course the customer's prerogative, but such customers
too are indicating just what kind of service they expect in the future.
Professional waitpersons will try to ignore such slights, and continue
to provide courteous, efficient service to every customer, but some
servers will definitely respond in kind. Rumors of terrible things being
done to people's food are largely exaggerated, but some servers will
undoubtedly refrain from doing any special favors for customers that
continuously refuse to tip or leave extremely low tips.
While tipping is not obligatory, failing to tip a conscientious,
professional server is considered an insult, and an incredibly small tip can
be more insulting than leaving nothing at all.