It is the default use case. But some team want to remove this strict rule and they sets this option to notify all team members about new review without reviewers. In your case we recommend you to set reviewers for the new review and notifications do not send even if option "Allow creating reviews without pre-defined reviewers" is enabled or you can disable this option.
Best regards,
Mstislav Sakidon
Devart Team
www.devart.com