Why login page in "backend" application isn't layouted?

In my Yii 2 App Advanced theme-based app, I have noticed that “login” page for “frontend” and “backend” applications are different.

The key (the only?) difference is that frontend’s login page is rendered inside page’s outline (as probably every other page). while backend’s login page is rendered without any layout. Even though that layout exists for backend views and is used in other routes.

Is this simply omitted because forgotten? Or is this intentional and I am missing some obvious thing (i.e. some security or design issue)?

The backend’s SiteController’s actionLogin() method uses different layout than frontend’s. Have a look:

public function actionLogin()
        if (!Yii::$app->user->isGuest) {
            return $this->goHome();
        // here
        $this->layout = 'blank';

I know. The question remains – is this intentional (some logic behind) or simply because someone decided that it should work like this (no security issues etc. behind)?