This isn't really a conscious choice to encourage guests to join. I think it's a matter of security. Members can only edit their own posts. With guests, you run into the problem of matching which posts they wrote with permissions. Essentially, for guests to be able to edit their posts, they'd also be able to edit the posts of other people, which really is anarchy.
Good point! So it was something inevitable.
In any event, my point was that some meaningful and significant differences between the options available to registered members and guests
are needed; otherwise, registration will turn to be pointless. For example, the fact that guests cannot send PMs is good, though my experience shows that it is hardly adequate.