Thanks for the heads up on this thread :-)
I’ve never heard the term “retrieval site programs” before.
If anyone (including bots, scripts, robots) is unauthorized to view content protected by s2Member®, that user/bot/script/etc, will be redirected to your Membership Options Page. The content from the site is never made accessible to anyone that is unauthorized; regardless of how the site is accessed (via HTTP, HTTPS).
Acessing a site via FTP (where the FTP user is authenticated), would bypass all PHP scripts, including s2Member (including WordPress itself of course too). Nobody should ever have access via FTP except you, or developers working with you. This is something that should be handled by a system administrator (or your hosting company).