Re: UK Perspecitve
I agree with you about Starbucks. But if the program in the article is the one I think it is, many deliveries amounted to doing grocery shopping and delivering the order to the customer. In the case of produce, fresh meats and so on, the "driver" ended up selecting the goods what went into the cart. I can readily believe that some customers were very appreciative of receiving very fresh, unblemished fruit and veg and so on.
And, to leave a cash tip would mean tipping in advance, since these goods were mostly delivered "drop and run" (particularly because of the intensive monitoring of deliveries - too lazy to hunt up the references), so the electronic tips were the most effective way for customers to properly appreciate good service.