Why are trash customers able to reduce tips

Why do people think it’s ok to reduce tips after delivery, you could just put down any tip then reduce is so your food gets delivered which is digusting and Uber doesn’t even compensate you. This delivery I called the person because their address was incorrect I decided to call them to meet and the door but thought id be nice and go hike up to their room, couldn’t find it so hiked down to the door handed them their order and they cut all my tip off, insane these people are able to manipulate drivers like this, trash human beings , but Ubers fault for not caring about the drivers