Oh, because the instructor is unsafe? That ought to be enough, don't you think?
Sean Sorrentino, the proprietor an An NC Gun Blog, has his first person story of training gone bad. Venture over there and read it, because it's an amazing cautionary tale.
Edit, 6/6/2013 2033: OK, so don't go. Sean has closed the post. I assume he got tired of the what was happening in the comments.