I think the addon tryed just to read and stack all items in your bags. Stolen items (not laundered yet) cannot be stacked together with non stolen items. e.g. ItemID of stolen tomato is 923456 and non-stolen item ist 123456. That doesn't match and needs no effort to be stacked.
So he just changed that for performance gain
After laundeirng the items you are able to stack them again, as the itemid is the same for "stolen and laundered" tomato and a "non-stolen" tomato.