So, the heists you do in the Thieve's Den reward you with treasure items that have already been laundered. Wouldn't it be infinitely more useful if the heists gave you the items still considered stolen, so they could be used to accelerate the Black Market Mogul and laundering achievements? It even says for the reward container that the contents were "obtained less than legally", so why are they already considered clean when you get them?
It would also give players a reason to do the heists (much less get that DLC in particular) after they finish their thieve's guild motif book.