Because she knew it's prohibited and it's not what we do and everyone seems to be taking their free speech dig at our prophet and I would assume this is no different, because there is literally no reason to bring that image.

However, I think they should have left the class and contacted her regarding this subject and firing her probably was an overkill and done to make the problem go away, though not sure why the student complained to the administration before contacting her despite her giving warnings to the students.