Why does everyone expect a tip?

In some jobs a large proportion of the pay is in the form of tips. A lot of the waiters and cabin staff on cruise ships are on zero salary. All the money they get is the tips from their pengers. The tipping system is not there so you can show your impeccable judgment as to how good the service is, it is there so you get service at all in a business which could not afford to employ staff at that level if they were on a full salary.