To me, you tip a waitress or waiter inside a sit-down restaurant for exemplary service. You know, when they anticipate your need and refill your drink, bring extra napkins without being asked, make sure special requests are fulfilled properly, expediting delivery of your food, especially when you have a child with you....things like that.
Since when are we expected to tip people for just doing their job? They're SUPPOSED to be accurate, speedy & courteous in a drive-through. I don't see McDonald's, Walgreen's drug store, the bank teller or dry cleaners putting a tip jar in their drive-through windows. I mean, really. Is it just me? What's next? Tip jars at cash registers at Walmart or the grocery? Tip jars at the self-serve check out lane or gas pumps?