Why do some Canadian stars renounce their citizenship to become American?

Now and again I hear about one of our stars becoming American like Michael J Fox, Pamela Anderson, Jim Carry, or Alanis Morissette and I don't understand the purpose. For one, they will always be known as a Canadian actor or singer. Becoming American doesn't improve their popularity in the states, most Americans probably already thought that they were American... the only people who hear about or even care for that matter, is the Canadian public; it's like a kick in the face to us. They publicly switch their citizenship and lose millions of fans over the border... for what???