Right or wrong, it's a fairly common stereotype: a merchant who insists on cash-only transactions is dodging taxes or selling something illegal (or both, I suppose).

On the topic of criminal behavior, it's also worth noting that doing business cash-only makes you a more tempting target for robbery; if you did almost all of your transactions via credit or debit cards, you'd have very little in the till for someone to steal.

Being a cash-only business also means they can pay their employees in cash - often because their employees aren't authorized to work in the country.

We hit that point a long time ago when having a large amount of cash with you became a justification for civil asset forfeiture.