How to allow multiple values on a civicrm contextual filter in drupal views
It's a limitation in CiviCRM views implementation.

If you are a developer and know views api, i encourage you to propose a patch for this. You can find all the code in drupal/modules/views/civicrm.views.inc or https://github.com/civicrm/civicrm-drupal

More precisely, the function civicrm_views_get_argument return a views_handler_argument_numeric for all numeric data type. You might want to try changing that to views_handler_argument_many_to_one. But it's really a guess : i don't know if it will work and even less if it will work for every fields.

Otherwise, still with some programming knowledge, you could do a drupal module that implements hook_views_query_alter but it's more a drupal question.