The issue is the possible in-person judgement associated with the tip. Traditionally a waiter will not look at your signed bill at your table. They didn't stand over you with a mobile POS system waiting for the tip. Generally those who receive cash tips do not count it in front of the customer. They will find out the tip amount, but not in front of your face.

But with the POS systems, its easy to tell how much someone tipped. Did they go left or right? Even if they turn around the system and don't stare at the screen, they can figure it out. They especially know when you put in a custom tip amount, which is assumed to be lower. Some even show the tip amount to the service person as they turn it back around. There is an pressure to push the center or rightmost amount out of nicety.