Why is McDonalds sweet tea in styrofoam cups?

I guess this isn't much of a serious question but more of just a curiosity on my part, but why is Mcdonalds sweet tea in styrofoam cups where as all of their other large drinks are in plastic cups? Is it supposed to keep the drink colder? I don't see why they would only give thermo-insulated cups to the tea and not other drinks. Or is the cup cheaper to produce seeing as sweet tea is almost their biggest selling beverage next to coca cola.