Whenever people shove a calculator into my face, I expect a scam. One of the oldest, used a lot in restaurants: 50+0 (showing customer the zero) + one coffee + ...
Apart from building 'trust', calculator plays important job of communication - people in places like Zambia probably do not know how to write in numbers in English (or are not very fluent) and also probably do not know how to say the numbers in English.
Being traders however they would recognize numbers readily, so they communicate via calculator by punching out the numbers. That is, calculator is used even when there is no calculation involved.
[Edit - Corrected the Roman numerals to English numbers]
People can calculate things in their heads, have you never bought something where there wasn't a calculator?
And as mentioned, calculator I have seen being used as a communication mechanism (You ask what's price? Seller taps on the calculator and shoves it under your nose without uttering a single word : 72.50)
The scam I talked about only works when adding up larger number of items (like a table of 4 at a restaurant) but there it is pervasive.
With honest traders I never had a problem, neither in Africa nor in South America: Calculator on the table, reset in front of me, slowly, step by step, add up the items so I have time to punch it into my own calculator/phone.
Don't know where you get this impression from; based on my first-hand experience, it is very likely wrong. If you are referring to people who are illiterate in English, fair enough (although even then they most likely can say the numbers in English). But then you would need to check the rate of illiteracy in Zambia (and their language of business) before making a blanket statement like above
Imagine Germany, Russia, Indonesia, China, S.Korea...