Not sure if this flow is the desired outcome of the plugin, but I think it seems like an unnecessary extra step.
1) User selects "Free subscription" from the register page
2) User fills out registration form for free subscription
3) Upon successful registration, the user then has to select the subscription once again (http://dl.dropbox.com/u/120937783/Screenshots/Screen%20Shot%202013-09-18%20at%2014.36.45.png)
Problems with this:
1) There is no email confirmation
2) The user must reselect the subscription they want after they have registered