Billing Block is not consistantly shown for paid events when using a priceset

I have reproduced this in the public sandbox. Try to register for the event at: http://drupal.demo.civicrm.org/civicrm/event/register?id=8&reset=1

If you choose 5 adult tickets, but "none" for the contribution then the billing block is not shown and the end-user cannot register for the event. ( even though the price set total is: $ 250.00)

The only way it works is if the end-user chooses a contribution such as "Granite $ 2,500.00" (or some other non-zero contribution amount) in addition to the adult tickets.