Incompatibility with themekey and perhaps all other themming modules

After tracing the code i realized the the following code overwrites themekey configuration for any user that does not have access to admin theme on all pages. May i know why you're unsetting custom_theme?!

  if (!user_access('access admin theme') || $admin_theme_disallow) {
    unset($GLOBALS['custom_theme']);
    return;
  }