One of the major benefits of REST is to define a interface that a remote client can access easily with the minimum amount of coupling between client and server. This is very useful when you do not control the client. This allows you to evolve your server interface without breaking existing clients.
The REST interface should really just be a thin layer over your validation and registration logic. In theory it should be easy for you to re-use that logic in your own website without going through the REST api.
Your website is not on a remote machine and you have control over both the client and server portion so you are not gaining anything by going through the REST interface.