i think it is alright to turn up without pre-registration. try and you may be admitted. i cannot imagine why the organiser will turn you away.
Because the auditorium can only admit X number of people. Expecting X-20% of the people to turn up, they overload abit more. Imagine X + 20% of the people turning up. You wouldn't want to be one of the last 20% to reach the avenue, and find that you can't enter the audi to submit your photos because it's too packed would you?