Strategies for Achieving Decoupling Between API Methods and User Interfaces

What are the proven strategies for achieving decoupling between API methods and user interfaces?

I'm working on creating a REST API service for the application. And I noticed that my API methods are strongly coupled with the user interface.

Let's assume that I have this simple entity in my app

public class User{    private String username;    private List<String> roles;    private Set<String> permissions;}

Now let's have a look at the user interface.

There are two pages in the user interface, one allows an admin to create a user by specifying his username and roles. Another page allows an admin to issue permissions.

This means that I have to create two API methods, one to create a user and assign roles. And another one for issuing permissions.

public class UserService{    public User createUser(User user){        // save user to db        // assign roles;    }    public User assignPermissions(Integer userId, List<String> permissions){        // find user from db        // assign permissions    }}

Why should I strictly follow the UI? In my opinion, it is more logical to create three API methods, one for creating a user, another for assigning roles and the last for issuing permits.

So, how can I achieve this decoupling?