Disabled registration form fields

On my “sign in” page there are two links, one to 'checkout as guest' (http://site.name/index.php?dispatch=checkout.customer_info&login_type=guest) and one to 'register' (http://site.name/index.php?dispatch=checkout.customer_info&login_type=register). Selecting either of these takes the user to this page: http://site.name/index.php?dispatch=checkout.customer_info which has a registration/user details form, but all the fields are disabled

Obviously this is intentional, but why? And, more importantly, how do you turn this behaviour off?

The register link in the sidemenu works fine (http://site.name/index.php?dispatch=profiles.add)

I'm having this same issue. It looks like a feature rather than a bug, but can't find where this is set. Each field is disabled, so no users can register…