You can also take a blacklisting approach, where it's open-by-default and users shall be able to restrict any domain to properly authenticated users only. That way, it is purely a security enhancement for those who want it (like me).
I demonstrated this behavior to eggsampler after discovering it quite a long time ago by messing around with HTTP payloads in their web interface - it's wild to me that FastMail will use the DKIM private keys from an entirely different FM account to sign your messages.
Unlike eggsampler, I won't be ditching them, but I hope that FM reconsider their policy eventually. That they have awarded themselves the privilege of a "green tick" on their own official emails while throwing everybody else to the wolves is slightly ironic.
