I see two questions here:

- Should database developers model business logic?
- Should I know how to persist data to the database?

**Business Logic**

- Business logic does not live in a database. It's a conceptual model which must be independent of other layers within your system.

- You can always replace one database with another, or you might even decide to use a NoSQL solutions to address potential performance issues. 

- Database developers can be involved during modelling process, but normally this is done by people with a complete understanding of a problem domain. In our organisation this is done by server side developers.

- Many think that database is a foundation of your application. Make poor foundation, and the system will fall. I disagree with this. I see database as a storage medium that can be always replaced.

**Persisting Data**

- You should know how to persist data to different storage mediums, including SQL and NoSQL solutions.

- Level of required expertise will vary with the size of the system that you are working on. 

- Small companies would expect you to have that knowledge, when larger companies would normally hire experts to address performance, scalability and security related requirements.

**To summarise:**

- Domain model is a foundation of your system, not the database.

- If you work for a small company on a relatively small project, then you should probably design the database yourself.

- If you work for a large company on an enterprise system, then it probably makes more sense to pass work to the domain experts.