Your understanding of what constitutes a billable item -- is what I believe. They could either add a clear column box to at least make it one click. Ideally they just add an attribute on the item setup for any item to determine whether or not this is 'billable by default'.
One would never bill at cost or in the case of travel or hotel it typically involves pass-thru. I could also see getting prints from a printer -- and passing that thru as part of 'incidentals' expenses.