A clear and strong message, before you even apply to go the Vipassana retreat is that you are agreeing to try the meditation, as taught, while at the course. There are stipulations when you can leave. Like not at all, before you finish the course. Some people do leave early, you are not kept by force.
This is emphasized because there is an understanding of how important rituals, mantras and symbols are in a person’s life. There is no judgement placed on these, but you understand, before you sign up, to set them aside for the time of the course.
Goenka Ji explained this in many ways. The easiest to illustrate the importance of this is the use of mantras while meditating. He acknowledged the advantages of these practices and explained some of the benefits of using them. Then he requested everyone cease and desist.
You see, Vipassana teaches you how to focus your mind on your body. If you are trying to simultaneously repeat a Mantra, or visualize a God, and focus on your body, you are hindering your own progress. You are trying to teach your mind how to focus on your body, not a word or an image.