8 – Why can’t I programmatically update checkboxes/radios that are not set?

I am working on a module in which I subscribe to an event and I am un/checking user account checkboxes accordingly. However, I recently noticed that I can’t check a box if none of them are checked. Why can’t I do it? Below is an excerpt of my code. I hardcoded the loaded user just for the sake of asking the question:

<?php
$earsiv_aboneligi = Drupal::entityTypeManager()->getStorage('taxonomy_term')->loadByProperties(('name' => 'E-Arşiv Aboneliği'));
$egazete_aboneligi = Drupal::entityTypeManager()->getStorage('taxonomy_term')->loadByProperties(('name' => 'E-Gazete Aboneliği'));
$user = DrupaluserEntityUser::load(2623);
$user->field_abonelik_turu(0)->target_id = reset($earsiv_aboneligi)->id();
$user->field_abonelik_turu(1)->target_id = reset($egazete_aboneligi)->id();
$user->save();