Boule de Suif shows that she is morally superior to her traveling companions because she shares her food with them at first because they did not think to bring anything. She refuses the advances of Cornudet and of the Prussian Comandant on principle (refusing to sleep with the enemy) rather than because she is a promiscuous courtesian. Finally, she does submit to the Prussian to secure the freedom of her traveling companions.
On one hand, Boule de Suif has lived her life bringing utilitarian pleasure to a vast number of people. On the other hand, she has trouble using the same skills to bring to a life a different kind of utilitarianism, namely freeing her companions from the Prussians. Maupassant effectively uses promiscuity to unleash a cornucopia of moral confusion. (http://www.answers.com/topic/boule-de-suif-story-3)
To show their gratitude to Boule de Suif for saving them from the Prussians, the riders in the carriage refuse to share their food which they have packed with Boule de Suif. They shun her as beneath their class even though they owe their freedom to her because she submitted to the Prussian commandant's advances and they are free to travel.
This is an example of the naturalistic style of writing. Boule de Suif's actions and dialogue show how the forces of nature conspire against her character.