But that's a different situation - the poster said that he asked the server what amount to tip, and they told him what to top. What you are talking about is a preprinted 'service charge' which applies to every diner, which is common many places.

Ancien Maestro, I don't think that it is 'sociological'. It really isn't that difficult to figure out, either. I think that you were given very good advice before your trip about what to tip, and how much to tip, from locals here on FT.

Think of it this way:

A person from Germany goes to Honolulu, and eats at a restaurant. The bill is $28.90, and they leave $30, 'because that's the way we do it'. One could say that is disrespectful, rude, or even ignorant, right?

Now someone from America goes to Germany and the bill comes to 28,90 Euro. They leave 35 Euro, 'because that's the way we do it'. One could say that is disrespectful, rude, or even ignorant, right?

But it is actually worse in that situation. In the former, only the waiter was impacted. In the latter, the North American tipping culture is used in a country where 20% tip is not the norm. Unscrupulous servers will try and extract 20% from the next diner who appears to be from North America, when they ask, just as it apparently happened to you. That's why I think that Rick Steves addresses it on his site, especially because a lot of his followers tend to eat at the same restaurants, where I suspect that this is quite common to try and extort the unknowing tourist for a larger tip.

This is how such things spread, and it's why people don't like it when someone disregards local cultures and norms. It's also many times embarrassing for the person on the receiving end of the tip.

And I say that as a former server and someone who worked in positions where tips were received.