The question is rather why wouldn't they? Starburst is easily one of the most popular slots to ever grace the internet. This popularity is enhanced through casinos offering free spins on the Starburst slot by signing up.
Casinos know the appeal of the Starburst slot. The fact that everyone wants to play the slot makes giving away free spins upon signing up a good trade off. What this means is that the casino gives away some free spins and in return receives a great number of new customers. It is also a way of online casinos trying to outplay their competitors by offering free spins on a slot that they know people love! Starburst is that slot so there is no surprise here.
In total you can get more than 100 No Deposit spins. Find Starburst no deposit spins here.