Instead of automatically importing all existing WordPress users to a preset list on plugin activation, please make this an optional and manual process.
It could be nicely integrated with the existing Import options like this:
  • Paste the data into a text box
  • Upload a file
  • Import from MailChimp
  • Import WordPress users
No need for a undeletable list "WordPress Users" or weird issues with unconfirmed subscribers that cannot be deleted, because they are still WordPress users.