If you look at the photo, the diet Cokes had a stir stick in them, while the regular Coke had no stick.
Edit: the more I think about the lid thing, I wonder if they are actually telling the truth about this being a waste reduction thing. I seriously doubt the cost savings is even a penny a drink. Though I bet they will eventually roll it back and do something like "lids upon request" when enough people are spilling their drinks into their food.