Why don't people say 'Roubles'?

I've noticed that language learners try to avoid saying amounts in their own currency, which my literal brain finds a little bit odd.

I was at a cafe earlier today and the barista quickly took the opportunity to practice her English (which is commendable). I only said "americano pazholsta". And then she wanted 3 pounds off me. I look confused. I'm sure I heard the 'd' sound for dollars before she settled for coins.

This is just one example of many. There's a god chance the foreigner has brought some of the local currency and probably would have researched the name of the currency too :)

I'm sure in Poland they don't have the same issue asking foreigners for Zloty, or maybe they do??