Is it customary in the United States to tip when picking up carry out food from a restaurant?
Even the people who argue that they packaged it and "got it ready for you", they are selling you a product and that is FOOD. Should you tip the person who puts cereal in boxes, or pours milk into cartons?
Yes, they had to package the food up, but it's not like that was an option - they put the food in a to go container because how else do they sell food to you? No one brought you food on a plate, no one filled your drinks. no one delivered anything to your house. You already pay the premium on buying prepared and cooked food, why should you have to pay to pick it up yourself?
Tipping has gotten ridiculous, to the point of carryout only restaurants providing tip jars - their job is making food and selling food. If you start to tip carry out orders, might as well start tipping baggers at the grocery store.
These are relatively low income people and everyone appreciates any kind of bonus. They still have to work to prepare, package, and bring the food to you. some servers only take care of the carryout counter. In many establishments people share tips, including dishwashers, cooks and bar tenders.