You can never know what source, or rather area of business the dirty money comes from. They could come from theft. Would you like to buy cheap gold of high purity knowing that it was stolen and then melted to hide its origin?
Official stamps might make you feel warm and cozy inside, but this is a longstanding practice. For example, the US government is long known to have melted down personal effects from the German occupation into official US treasury gold bars. This is basically how all conflict gold stolen in wars is treated. Money is treated as money, in other words.
True, but whether taking something from a country that lost the war is theft is a matter of opinion. Japan was being occupied after the war and the occupants did not think of themselves this way. In their minds they were peacekeepers. How much money, gold, oil was taken from Iraq? At the same time those little bank thefts, account hacks and other white collar crimes are always painted like the worst theft that needs to be weeded out as it destabilizes the system. The world is strange indeed.