Joan is able to inspire such admiration, that she launches a movement which eventually unites a country, shifting its entire power structure in the bargain. Even the men who put her to death can't help but respect her courage and tenacity. Her spirit was so strong that it continued to inspire for hundreds years after her death. She became a symbol for generation after generation. Eventually, admiration for her grew so much that the Catholic Church made her a saint. Saint Joan chronicles the life, death, and legacy of this inspirational figure.
To Charles and his followers, Joan is just as much a tool of political convenience as she is a source of admiration.
The fact that Joan's heart doesn't burn is symbolic of the admiration of her spirit which is still with us today.