But, as with table service (the assistance, order taking, getting every little condiment/addition when asked, clean-up), the special take-out hardware (containers, plasticware, etc) are added costs above and beyond the food. If it's a place I go to occasionally, now and then I'll chuck a buck into the "tips" jar. Generally not, though, as IMO there's no comparison between relatively inexpensive containers/plastics and a full-service waiter at my beck and call. So far as I can tell, that's tipping about 10x more frequently than the average carry-out pickups that I've seen.
That's a good reason, though it might not be your reason, or even acceptable to you as a good one. But, there it is.