According to wisegeek.com the average American spends about $2,700 on eating out each year and although I don't have that number further broken down by restaurants in which a tip is expected as opposed to fast food restaurants in which a tip is uncommon, that dollar expenditure by Americans is in aggregate in the billions of dollars. In North America, tipping at sit-down restaurants which offer table service is expected, and in fact, based on the salary of the wait staff that serve you in which the majority of these servers make less than minimum wage per state law, it is mandatory for these servers to receive good and consistent tips in order for them to make ends meet. What is surprising about this whole thing is the tip that you provide to your server is neither something that you are legally compelled to do, nor is consistent from table-to-table nor person-to-person nor dining experience-to-dining experience. You, the consumer, have absolute discretion, subject to social pressures, to tip the amount of money that suits you.
While it is suggested that your tip range should be in the 15-20% vicinity, tips can widely range outside of these boundaries, depending upon the dining experience, your financial situation, your mathematical skills, and your personality. I believe that the restaurant is doing you and their servers a favor when the bill is presented to you with suggested tip percentages of 15, 18, and 20 percent already calculated out, since there are such a significant amount of people that are either math deficit or excuse proficient when it comes to paying an appropriate tip.
Although tips have been a standard in America for a number of years, tipping and the compensation of servers in other countries does vary considerably from the American standard. For instance, in Japan, there is no tipping permitted, and in many European countries tipping has been replaced in the most part by "service charges" in lieu of a tip, although a smallish tip of 5% or so is not considered out of the ordinary.
But just because tipping in America is the norm, doesn’t mean that the model should be continued in its present form. The main problem with the American model of tipping is the fact that the amount of money that a particular server will make is unnecessarily inconsistent based on the consumer having the power to withhold appropriate tips from servers as a form of punishment, ignorance, or cheapness. A better model would be akin to the European model in which a restaurant will make it clear to its patrons that in lieu of tipping the wait staff, that a service charge of 18% has been added onto the check for all table service (but not take-out) and additionally that no tipping will be expected from sit-down diners. This model is both simple and straightforward, additionally; it allows the restaurant to take better control of the distribution of the monies received from the service charge so as to fairly compensate cooks, busboys, hostesses, and other members of the restaurant, should management of said restaurant be so inclined.
The only real perceived negative of the service charge replacing the previous tipping policy is that some consumers might resent this new program, that is why it is important not to make this proposed change mandatory for all restaurants, instead make it a choice by the restaurant management itself and let the market sort it all out. As for the wait staff, they too will have a choice, and I suspect most of them will prefer the new policy of a service charge, which appears both fairer and more consistent.