[arsenal]

Becuase the majority of restraunts (in the US anyway) pay their servers minimum wage. That amount depends on what state you live in. Washington State min wage is about $7.00, in Lousiana servers get a base salary of $2.50. So in a sence, you are working for your tips. If you don't get any tips, you don't make any money.

Also, you don't tip the worker in MacDonald's becuase they generally don't do anything. They take your order, give you your food, and that is all. In a sit-down type resturant, there more interaction between the server and the clients. The server is there to wait on you. The person taking your order at a fast food resturant is not there to wait on you.

If the service is bad, then don't tip. Its not manditory (unless your english in Quebec) to tip your server. But alot of servers work their butts off for you, and you don't even notice. They take alot of crap that isn't their fault (ie: cook screwing up).
If the service is good, the tip is the reward. Service is bad, no tip is the message you send.