I would guess that there is a reaction between your saliva and the ingredients that causes your own saliva to become the "juiciness" that you refer to. My favorite is the cherry one.
Starbursts have a lot of citric acid, which induces salivating reactions, in them. When you eat one, the citric acid causes saliva to be produced and mix with the starburst. The sugar and corn syrup then dissolves into your saliva and you reabsorb your saliva, giving you the illusion of juiciness.
After they harvest the skittles from their rainbow, they have two sumo wrestlers squeeze said rainbow between themselves. This produces the juice needed for starburst. :-)
Yuk bro, yuk..
I'm eating one right now (strawberry ofc)
