I don't think that money launders uses gambling sites to move money from one place to another. They can use mixer services and exchanges too for this purpose where there no hassle of wagering etc.
Yes, I agree but money launderers could play im casino. We do not know that they too like to gamble but it will not be called as money launderers when they gamble. They called as gamblers. In my opinion about money laundering is to transfer money by means if concealment illegaly obtain as per google dictionary. Now if they will be using a gambling platform then high chance they could get compromise and even the gambling casino to be suspected from this kind of activity to which gambling platform will not allow. If I am the owner of a gambling platform then for sure I will not allow any users to use and abuse the platform by this kind of acts.