The subscription you are referring to is currently free. 6 months ago we began soliciting feedback from the community regarding pricing [1]. You might want to take a look at the feedback we have received and the motivation behind adding a premium to the AMI images. We have yet to come to a decision though, so feedback is most definitely welcome.
In a nutshell, the idea is to fund development. When you use Amazon EC2, you are already charged on usage, so the idea is to add a reasonable mark-up on those fees so people who are using TurnKey give back a little bit into a pool which helps support the project and fund future development.
We believe this is a win-win for the community, because a premium would fund development of open source appliances rather than expensive and closed proprietary alternatives. To the casual user a few bucks here and there would hardly be noticed on their bill but multiply this thousands of times over and suddenly we have the resources to do much more for the project's users.