If you offset the pump to one side of the bilge they take the outlet up to deck level then accross and through the opposite side of the hull should aviod syphoning. Often a shorter and therefore more efficient rout than through the transom. As an alternative ther is no reason it cant go either through the deck or the cabin side. Lots of commercial vessels do this and I suspect its not done on yachts for fear of staining the deck rather than because its a bad place to pump to.