Why do I not have the ability to customize roles & additional permissions settings when adding an Enterprise Workgroup in a project?
When you "Add All Current Users", you're adding individual users as they exist in the workgroup *at that time*.
You aren't really adding the workgroup entity - only its contents. Hence, you have the option to update the individual users right then and there (since there won't be any automatic behind-the-scenes changes to the project's member list based on future changes to the workgroup).
When you "Add Enterprise Workgroup", you aren't just adding individuals - you're adding the workgroup entity itself.
Thus, any changes to the workgroup also affect the membership on your project.
All members of the workgroup will assume the default role specified in the Add Workgroup wizard screen.
All members of the workgroup are handled the same way (with that default role) because that's also the same role that will be applied to new members that are added to the workgroup by the admin at a later date.