6RD only makes sense in a single routing domain. 6RD is simply 6to4 using ones own IPv6 space instead of the global 6to4 range (2002::/16) to form 6RD IPv6 addresses.
So it makes sense for an ISP who wants to provide IPv6 access to its users while it upgrades its infrastructure to IPv6 capability, or a corporate entity, etc.
But it doesn't make sense for a tunnel provider like HE, where all its users are spread all over the globe. They'd have to get a a huge allocation of IPv6 space to represent the entire IPv4 internet, route it to a central point, etc.
I'm also not sure how HE could provide a test for a user's implementation of 6RD, since the IPv6 traffic would just come from an IPv6 range allocated by the user, and the addresses would look like any other IPv6 traffic. Plus, setting up 6RD at an ISP or enterprise is a little beyond the scope of what HE is doing the cert thing for, which is really fairly basic IPv6 networking stuff.