It works with the same User model and UsersTest unit test when I place the following code into app/filters.php:

Event::listen('eloquent.deleting: User', function($user) {
    // sdv_dbg('eloquent.deleting: User');
    // sdv_dbg('User',$user);
    $user->userRoles()->delete();
});

Why does not the next one works? Is it a framework bug or I do not understand something?

    public static function boot() {
        parent::boot();
        static::creating( function($user) {
            if ( !$user->isValid()) {
                return false;
            }
        });
        static::deleting( function($user) {
            $user->userRoles()->delete();
        });
    }

Do you have any foreign keys on that table or from that table?

It could be the DB is enforcing a foreign key and won't let you delete a parent without destroying the children first.

No, migrations do not define DB foreign keys. There are only Eloquent "virtual" foreign keys in ->hasMany() and ->belongsTo(). I came from MediaWiki background, where foreign keys are not used. Unit tests related to adding / deleting relations pass completely. Only ::deleting() inside ::boot() does not work. I am using 4.2.8.