An European's perspective on tipping is nicely covered here: https://www.youtube.com/watch?v=vBTfj2lN6sQ

Basically, "The bill came to $20 therefore you asked me for $20 and as per your exact request I gave you $20. Just like you asked. And I'm somehow an asshole for that."

Yes, he also covers "waiters are underpaid", etc.