So I believe this may be an edge case that just hasn't seen much consideration...
I assume this is a combination of having 'Enforce Single Membership Plan' enabled along with membership plans on the User Registration form...
Radio box selections come up on my User Registration form to allow for the selection of a membership plan...
The problem is, I don't have 'Enforce Paid Membership at Registration' enabled - so a user does not have to select one of those radio boxes, but if they do, there is no way for them to change their mind and not buy a membership, short of reloading the registration page.
PS: Because I'm using email confirmation, I don't think it feels all that correct to have my memberships on the user registration form anyhow (after all, users end up at their user page, with purchase membership links, as soon as they confirm their email address) and as such, this isn't particularly applicable personally, but I figured I'd point this out, as it seemed a bit odd.
Vector,
You can turn off the 'Show on Registration Page' option if you don't want the membership plans to show up on the Registration page.
I agree that adding an N/A radio is a good idea if 'Enforce Paid Membership' is turned off, and I will add this to the feature request list :)
Sincerely,
Leighton Whiting