The email address field you are referring to is part of the Employee interface. It is not tied to a contact record and is stored as part of the activity log. The behaviour is working as designed as the person logging the case, the Employee, would be tasked to obtain accurate information about the person who they are logging the case for.
An idea could be created to do one of the following:
- Create an incident email address field that is filled/stored only by the employee interface and can be leveraged as a field to mail against
- Have a new contact created based on the email field on the Employee Incident form (might not be considered due to the dangers of flooding the contact table with extraneous and false contact information)