"In about 15% of cases, customers leave no tip, and DoorDash pays the entire amount, the company tells us. (The default tip is usually set to 15% the app.) ... In the next roughly 45% of cases, customers contribute less than half the money required to meet the minimum quoted fee–called the 'guaranteed amount'–leaving DoorDash to chip in the majority of the fee."

So under their model, if you tip well, you're subsidizing people who don't tip at all.