They point is making it hard for the tax evaders to spend their cash (which they cannot deposit in the bank since it's way more than they officially earn) in law abiding shops.
Thank you for the clarification, Ultimo (so there is at least two of us), but the rationality of the measure is still doubtful.
This starts making half-sense - it is not to curb the possibility that the merchant evades, but to hinder spending earning coming from evasion. Unfortunately, that money that may come from evasion is the same money that comes from normal earnings: so the damage is spread, not surgical. For example, now Men (is it just the two of us?) in Spain cannot purchase, say, a Macbook (the threshold there is reportedly at 999€).
It seems like one of those generalizing measures like "Populations labelled as irresponsible socially extroverts as an aggregate will have individuals bound to confinement in case of epidemic", which even when they work as an effect involve massive lateral damages.
Like for other measures, especially in the past few years, measures are taken and justified though a broad vague irrational paternalistic shout ("For the children"), and we are left with the unduly burden of trying to reconstruct the logic (if any plausible exists).
Most of them introduced them to decrease tax evasion, not money laundering.
You can still have that much money in cash, just not pay with it one transaction.