price sets on front end contribution pages don't display correctly for extension processors

Currently reproduced here:

http://drupal.demo.civicrm.org/civicrm/contribute/transact?reset=1&id=2

if you install sagepay as an extension & set it up as a payment processor (with made-up credentials) and then change a membership contribution page with priceset to use it the live page doesn't display the fields of the price set.

I haven't tested 4.2 or with non-membership pages but have reproduced with several extensions incl google checkout, sagepay & omngateway. Switching to an inbuilt page fixes.