The reason they're using "mechanical contraption" is because:
1) some of them they think a calculator would give an incorrect answer, whereas mechanical contraptions never lie, cause you can actually see the process of counting
2) most calculators need batteries and when you'll get used to it and battery will finally die, you won't be able to service the customer

If you stay long enough in shops like these, you can see them use the calculator first and then switch to abacus to check the calculator's answer.