Why do you have to fill in your tip amount?

I don’t think I really understand your question—or maybe you don’t really understand how American tipping works. Except under limited circumstances (big groups, for instance) tipping is *expected* but also *voluntary.* If you are paying with a card, you have to write in the amount you want to tip. For table service, 20% or more is standard. You also have the option (even when paying by card) to leave a tip in cash. Your server might actually prefer a cash tip, if it’s feasible, even if you pay the rest of your bill with a card, because of… reasons. But if you *don’t* write in a tip at all, you are kind of a terrible person, from an American point of view—you are spitting in both the server and the business, each of whom are penalized (in different ways) by your decision. Is this a good or fair system? No, obviously not. But it is what we have, and refusing to go along with it is just hurting the people who are lowest in the pecking order and have the most to lose.