If allowed I don't see how it's impossible to collect one's own food from the kitchen or counter. There are places which work that way.Apples and Oranges I'm afraid, speaking as a former waiter. Petrol attendants perform a task you could do yourself (as you have indicated) ... waiters do something you can't do yourself and by all means, don't tip if they do it badly.
Also, Riaan's post claimed that he doesn't tip as petrol are paid to do a job. Waiters are also paid to do their job.