My son has been terminated since 2008. Reinstatement requires a preexisting condition exclusion period. Why?

After my son graduated high school in 2008 he was terminated because he chose not to continue his education. I have been allowed to reinstate him and now we face a 365 day pre-existing condition exclusion period to satisfy. How is this fair or legal? I am with the same company that had him covered before. I have a CERTIFICATE OF GROUP HEALTH PLAN COVERAGE. He has had a great health status with no trips to the emergency room, along with no prescriptions prescribed. I don't understand!