Um, there's nothing "shameless" about it. Romeo and Juliet was acknowledged as being the source for the story. Opera's throughout history had other sources. Do you think Mozart's Le Nozze di Figaro or Rossini's Barber of Seville were original works? Two plays involving the same group of characters by Pierre Beaumarchais were the original source of *both* operas. Don Giovanni? Don Juan of which there had been a number of plays and operas written before Mozart wrote his Don Giovanni.
And I would point to the improvements over Shakespeare in WSS. There are no suicides. Romeo does not find a sleeping Juliet and, thinking she is dead, commit suicide. Juliet, upon seeing Romeo's lifeless body, does not plunge his dagger into her chest. Instead, Tony, upon hearing Maria has killed herself, runs off screaming for her and looking for her whereupon he is shot just as she arrives on the scene. He dies in her arms with her rocking him and crying out the words to "Somewhere" and screaming at those around them "Don't you touch him!" She has died in her heart but lives on to carry the memory of Tony with her. He has died physically. But neither "die" by their own hand.
And what makes WSS so perfect is the incredible music. "Somewhere", "Tonight", "One Hand, One Heart", "Maria", "Something's Coming". These songs are classics, and rightfully so, regardless of their source musical or its inspiration.