To be made properly for a restaurant, sweet tea has to be sweetened with simple syrup, not sugar. Fresh-made sweet tea does not keep well; if it sits for more than 4-5 hours it starts getting cloudy.
I'm guessing that Universal serves so many guests who have such a wide spectrum of taste that they find it saves labor to just let guests sweeten their own tea w/ table sugar.
Southern Traveler's Trick: make your own simple syrup and carry some in a small pocket bottle. Here's a recipe:
2 cups granulated sugar
2 tablespoons light corn syrup
1 cup water
Mix sugar, corn syrup and water in a medium saucepan and bring to a boil. Cook for 2 minutes. Remove from heat and cool completely at room temperature. Pour into a covered container and refrigerate. Can be stored indefinitely in refrigerator.
Makes 2 cups.
Simple syrup doesn't need to be refrigerated if carried for short periods, but you don't want to store a big container at room temp., it will get moldy over time.