Why does the Starbucks mermaid have two tails?

It was the intention of the original Starbucks founders to use the mermaid/siren to represent the seduction of the sea going mariners. In continuing with the metaphor, they were simply replacing the sirens seductive songs with the their tasty irresistible berverages. It is based off Moby Dick and the character Starbuck who was a sailor who loved coffee. And sirens were seductive to sailors they would jump out of boats once they heard the sirens' songs. So it basically means that it is irrestible.