then why is Warren Jeffs being called a prophet?
The Mormons have a head of the church, which they call "The Prophet".
It is an official position, and they feel he is their messenger from God.
Kind of like the Pope to the Catholics, but not really (the Mormons are Christian, but they deny the papacy and the authority of the Catholic church).
Some Mormons splintered off and left the church over the Polygamy issue (the church officially rejected polygamy, because Utah was not going to be granted statehood if they did not, and some members thought they were compromising their religious beliefs by doing so and left the church).
Jeffs is seen as a messenger from God delivering the truth to some of the people who have left the church over this issue, therefore their Prophet.