I also don't understand why they care.  But several of the tour operators we used on a recent cruise said they wanted cash in US dollars and they had to be very clean with no marks on them.  So before the cruise we spent 15 minutes with our very patient bank teller getting the exact denominations of bills we needed, since the tour operators can't give change, and trading back and forth bills to get clean looking ones.  No tour operator or merchant on our trip to Chile, Argentina, or Uruguay refu