What do magpies represent?

Magpies (black birds belonging to the crow family) represent duplicity in The Joy Luck Club. The author strategically places stories and mentions of magpies near real-life occurrences of deception and hypocrisy. This helps the reader to feel the true dimension of the main characters' sense of betrayal, their disillusionment with all the values that they were taught to cherish but grow to find repugnant.
For instance, An-mei tells her daughter Rose a story that her own mother told her, namely that a turtle ate her tears and laid eggs which hatched as magpies. The point of the story was to convey that one person's sadness creates another person's joy. This is a positive interpretation of the story. However, in real life, one would argue that Rose, An-mei, and indeed all the women in the book are entitled to their own happiness, and should not have to settle for the so-called comfort of providing someone else with joy.
Why does the author use magpies to convey opposing meanings? In Western tradition, magpies are considered a bad omen, yet in Eastern tradition, they are considered good luck. The difference in global attitude towards the magpie is a symbol of contradiction and deception. One object, one story, one life, can be viewed from multiple perspectives, and to limit it to one is, in fact, a deception. This is something many characters in the book face, as they find that what they were taught to expect and value in life is not necessarily what they find to be truly important.