Fatburger tip???

Hello I tried fatburger 2 times, take-out and eat-in in different branches in LA. Yesterday i ate fatburger in the store and they showed tip screen before payment. I was confused but chose to pay a tip because i have no idea what they will do in my burger. ( i saw some videos how nasty people can be when you don’t pay a tip). Fatburger is fast food like inandout burger and is it normal that they show you tip screen when you eat in the fatburger store ??