I am almost positive that there are provisions for students in our field.
I think that otherwise, it could be two-fold:
A bunch of kids running around can be obnoxious, and it would be an absolute goldmine for older kids who just want the goodies and won't ever buy a damn thing.
Bottom line is they are charging exhibitors a pretty penny and essentially promising them a good target audience. Pewrhaps they don't want that audience diluted with too many non-industry "plus-ones"