Not so fast.
The backend (i.e. Rails) still does almost everything it used to do: validations, access control, session management, data crunching, and everything else that you can't blindly trust a client to do. The real difference with client-side applications is that instead of stitching together view templates and sending back HTML documents, JSON objects are sent back for the client to represent.
More (sometimes duplicate) stuff is added to the client - things like client-side validations, and all the business logic and template code for the purposes of presentation. But no one in their right mind would let their backend blindly consume whatever it gets and persist it without question.
There's still plenty of responsibility for the backend.