> before you have received service
One supposed origin of the word TIP is "To Ensure Proper Service. Ask people who are old enough and they'll tell you about how a tip was put on a table at the beginning of a meal. Today that's extremely rude, but it was common in certain areas.
Separately, I really don't understand this tipping economy at all. Aren't tips supposed to be based on quality of service? Why do they ask you for the tip amount before you have received service? They should just charge a fair amount and pay drivers a fair amount and not over-complicate the checkout process.
In general I absolutely hate "how much do you want to pay" type interfaces. I really hate thinking about that stuff. Just give me a number and I'll decide if I want the product or not -- that's how the rest of the world works.