Most of the web applications use some kind of authorization and authentication subsystems to allow its users to access private information of the application. However, the authentication process can be tricky if you don't have a clear idea about how to implement it as Backbone does not provide a hint about how to do it.
Backbone is authentication agnostic, which means that its does not provide objects or tools to implement an authentication strategy. The advantage is that Backbone is not coupled with an authentication mechanism and the disadvantage is that you should care about it.
As Backbone was made with REST APIs in mind, you will have to deal with the authentication mechanisms that are common in that kind of APIs. That's a good reason why Backbone does not impose or provide tools in order to authenticate users.
Another thing to keep in mind is that REST APIs should be stateless, which means that they do not keep track of the requests that you previously made...