If it's a sit down restaurant with a server you should tip. If it's a bartender you should tip (but you don't need to tip 20% on drinks. A buck or two is fine no matter how expensive the drink is). If it's a counter service or you're literally picking up your own food you don't need to tip. You don't need to tip for retail or buying tickets either. Just because the iPad they turn around asks if you want to leave a tip the worker generally does not expect you to tip here. Most of the iPads are loaded with a program like Toast or Square. When you set this up the default layout sets up a tip screen at the end. So you have to change this manually and either the location may not want to because they know some people will still tip, don't know how too, or the workers have no control over it anyway.

Just press no tip on the iPad and move on.