Yes, I know exactly what you mean and had the same thoughts before. I believe checkmk “thinks” if nobody is on the contact list, then nobody needs to be notified. Maybe this is kind of legacy code as the first notification method that jumps to mind is email. I’m pretty sure checkmk started with that. Meanwhile we have other notiication methods which by themselves do not require a dedicated recipient (like your POST request) but checkmk apparently requires one.
Try checking the Notify all contacts of the notified host or service. box and make sure at least one user is a contact for the host in question. I created a technical notification user for that purpose and made him a contact for everything. He doesn’t even have a password set so I assume he cannot login but because he’s a contact, he gets notifications.