Because You can sell anything. For example if you intermediate a purchased item it would be unreasonable of Petkafy to block you from creating a sales order with such item number. Then You would probably create another item and add the purchased part to its Bill of Material, but that is not very efficient. Therefore You can select anything for sales from purchased to semifinished to finished items, because sometimes You just might want to sell them.
Why do I see also purchased items when creating a sales order?