Maybe they want an idea of how many people will attend. If people just needed to type in their names and e-mail addresses, it could be that a lot more people sign up than attend. Also, they might have space limitations, and so they might have to turn people away. Normally rationing of conference attendance is done by charging a fee for a limited number of slots. Maybe they wanted to base attendance on interest rather than willingness to spend.
But it would be nice to hear an official explanation.