Why does AuthorizeAttribute redirect to the login page for authentication and authorization failures?

[Authorize(Roles = "CanDeleteTags")]
public void Delete(string tagName)
{
    // ...
}

[AttributeUsage(AttributeTargets.Class | AttributeTargets.Method, Inherited = true, AllowMultiple = true)]
public class AuthorizeAttribute : System.Web.Mvc.AuthorizeAttribute
{
    protected override void HandleUnauthorizedRequest(System.Web.Mvc.AuthorizationContext filterContext)
    {
        if (filterContext.HttpContext.Request.IsAuthenticated)
        {
            filterContext.Result = new System.Web.Mvc.HttpStatusCodeResult(403);
        }
        else
        {
            base.HandleUnauthorizedRequest(filterContext);
        }
    }
}

// User was redirected here because of authorization section
if (User.Identity != null && User.Identity.IsAuthenticated)
    Response.Redirect("Unauthorized.aspx");