Sorry for the late answer.
You are right, seems this is a bug in the TelerikMembershipProvider - the providerUserKey is not handled by the CreateUser method.
As we plan to release a service pack soon after the Q1 of the RadControls, we will include the fix for this issue as well.
We apologize for the inconvenience. You telerik points were updated for reporting this problem.
the Telerik team