US currency is accepted in a surprisingly large number of countries abroad. Just not in Europe proper. US dollars are even accepted in some European sovereign territories outside of Europe.

It is very convenient for Americans. Depending on the parts of the world you've traveled it is easy to get the impression that the US dollar is a sort of universal currency.

Which isn't to excuse the people in your story. It is pretty easy to find out if US currency works where you are traveling.

I've seen plenty of waiters, taxi drivers, etc., be quite happy to receive tips in USD in many countries where USD is not the official currency. In fact, I can't think of a single time when I've seen such a tip be rejected because of its currency.

That's quite different from trying to pay a bill (invoice) in USD in those countries.

In Romania, at least a few decades ago, tips from foreigners were expected to be in dollars. Tipping in Lei would be weird.