Oh yep, I know what your saying. I just feel they could do in a different manner. Here's how:
a) Track users via the update module (check the key used upon sig updates).
b) Add in a 'Do you wish to re-register' screen upon expiration, stating that it is free etc, and make it easy for people who just want it to 'work'.
c) Send out a courtesy email with a new registration key prior to expiration.
Maybe?