s2Member allows any WordPress user with a Role that allows permission to edit Posts/Pages to also have access to the s2Member Post/Page Restrictions Meta Box on the Post/Page editing page:

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If you want to customize this behavior and limit access to the s2Member Post/Page Restriction Meta Box, there is an s2Member filter that will allow you to limit access given your specific rules. You could create an MU-Plugin with the following code:

<?php
add_filter('ws_plugin__s2member_add_meta_boxes_excluded_types', function($excluded_post_types)
{
    if(!current_user_can('SOMETHING')) # If they are not allowed, exclude all post types.
            $excluded_post_types = array_keys(get_post_types());

    return $excluded_post_types;
});

In this example, you could change SOMETHING to any WordPress Capability (see WordPress Codex). You could also customize this code to use a different conditional that checks for something else that lets you control access to the s2Member Post/Page Restrictions Meta Box.