What is the prohibition in money laundering
According to Halacha one must refrain from wrongdoing and seek to prevent crime. Dealing in stolen goods is also prohibited as this is "strengthening the hand of evildoers since if he wouldn't be able to sell the goods, the thief would not steal." [Rambam Mishneh Torah Hilchot Geneivah]
A society where those who perpetrate crimes are able to benefit from their illegal gains has a problem. To use the Talmudic expression "the mouse is not the thief rather the hole is."[Talmud Bavli, Gittin 45a]. In other words, the opportunity for theft and benefiting from it is as problematic as the theft itself. Each one of us has a responsibility to prevent those with ill-gotten gains from benefiting from it. Clearly, money made from drug dealing and illegal arms trading would fall into this category.