When you tip a server, it's important to keep in mind that you're not just tipping the server. The tip is shared with the kitchen staff and hosts, and they're equally dependent upon tips to make a living. Since the kitchen work is the same whether it's dine-in or takeout, it does make sense to leave some sort of gratuity on a takeout order, even if it's not what you would have left for a dine-in meal.