Well the way i believe that it works is that box sales cover development costs like any other game so are needed, and in a game like this with a persistant world they need the fee to cover various wages/server costs etc. Obviously free would be nice but its damn near impossible for any MMO game.
Played it in beta and would buy it but im not sure it would keep my interest for a long enough time to bother though, might get hold of a copy in the future though depending on what they add