That's fallacious. There's no inherent benefit to making the challenge a surprise - a warning is just that, a warning. Not a way of saying "leave if you can't deal with rape images".
So you are saying that to become a Medical Examiner, I can bypass all the actual dissection of bodies or parts that is part of my degree path because I am traumatized by a dead body? If that is true, then I never want to have any surgical procedure again - ever.
In an educational context, the excuse isn't valid if it pertains to your education. It is where "leave if you can't deal with it" is a logical response.