'Include Additional Email Addresses on Auto-Response' feature does not work with Email to Case Premium
You select the 'Send response to all recipients' option for an auto-response rule, but additional recipients are do not receive the auto-response
Salesforce included the ability to include additional recipients on auto-response email messages in the Winter 15 release. However, this feature does not work when an auto-response rule is invoked by Email to Case Premium. Unfortunately, Salesforce did not provide a method to use this feature outside of the standard Email-to-Case functionality. As a workaround, you can replicate this functionality with workflow per this article:
Reply-to-All Auto-Response using Case Teams and Workflow
Note that this workaround will require Salesforce Enterprise Edition or higher.
This is a limitation of the Salesforce platform.