There are two decisions, each with two options, giving four outcomes:
- forced to be subscribed initially and cannot unsubscribe later
- forced to be subscribed initially but can unsubscribe later
- not initially subscribed, but can subscribe later
- not initially subscribed and cannot subscribe later
The fourth option is (as far as I can tell) useless and so it makes sense to condense these to a single option with three choices.
Also, with both the current system and with this change, if the subscription is not forced then you can check who is subscribed to a forum by clicking on the Show/edit current subscribers link in the top-right corner of the forum screen.
Not only will this show you who is subscribed, it gives you the power to unsubscribe a specific individual as well as un/subscribe all participants.
You are right that the enrolment interface with two lists of subscribed/unsubscribed students and arrows to shift between them is needed to make this fully flexible, but you can already to 80% of what you ask for.
Correction: you can do everything you ask for, as this last feature appears to be in Moodle version 1.4