Membership bug: User registration -> Use subscription

Bug:
If a new user signs up using the [subscriptionform] and only one subscription is enabled and that subscription is set as the default in User registration -> Use subscription, and they do not click on that subscription, their account is left as "Inactive". However, if they try to access any pages marked as unavailable to subscribers, they get a 404 error.

Resolution:
What function marks a member as active on a subscription? If a default subscription is set in User registration -> Use subscription, new users need to be marked 'Active' automatically.