Typically the POS desktops are talking directly to a server in the backroom. The server in the backroom is typically where a manager will check their emails (via Outlook), take training via a web site, etc. and it's also where the database for the POS client desktops is stored. Every night that small store server submits the data to a main server at the "home base".
So, if the virus scan is on the server (typically is), and the machine goes down, then the business is effectively closed. It's not that the POS machines had a virus scanner on them, it's that the server does since it's used as a work machine for the manager as well.
That's how one of the biggest auto part chains in the US operates. It wouldn't surprise me to see this elsewhere.