PAM (Pluggable Authentication Modules) is a system security tool that allows system administrators to set authentication policy without having to recompile programs that handle authentication.
The pam_listfile module is used to grant access to various services. For example, the Web Proxy app uses pam_listfile to ensure that only users listed in the web_proxy_plugin group are granted access. With Active Directory and Samba 4 allowing mixed case usernames, this limitation is a wee bit problematic. Keep in mind, PAM itself is not case sensitive, just the pam_listfile module.