AFAIK, the government does allow the exchange of currency, it just mandates an unrealistic exchange rate at which there is practically no supply. However, there is a lively black market that is tolerated (probably because it's cash based and thus does not scale but serves average people well enough) - newspapers publish the rates, traders operate quite openly.

In an interesting validation of basic economy, the black market rates are much closer to the official ones in rurual tourist hotspots, presumably due to the much bigger supply of foreign currencies.