The Maine Lobster Promotion Council is as bewildered by your question as we are. Sue Barber, of the council, wonders if you seasoned the lobster tail or drizzled melted butter on it? "Could they [you] have thought that ‘you add salt to the water when boiling or steaming a lobster,’ so they [you] did the same to the tail meat?" she asks.
Unless you seasoned it or the sneaky people who sold you the lobster tail did, there is no good reason why your lobster would have tasted salty. Taken out of the salty sea, steamed in a pot of seawater and covered with seaweed, lobster meat still tastes sweet and fresh, but not salty.