It's illegal if the fact that a service fee would be added wasn't shared before ordering (on the menu / by the waiter /...)

Possibly. Local laws vary heavily, and could limit hidden fees like these. If the franchise is in one of these places, but the parent chain is not, it could easily be implemented despite being illegal. It's a similar case if the local operator didn't have the required notices in the required way, since it would be done separately. Not necessarily out of malice, but a ton of places simply do not run a tight ship. The receipt is absolutely not the place these notices are required; that's just a convenience.

It's also possible that the POS has a bunch of options that can easily be set by management without involving lawyers. A required tip (often for large groups, but not always) is an easy use case for this. So are the various messages, including the tipping scale, or adding a promotional QR code (e.g. scan the code to fill out a survey and get $5 off your next visit)

In any event, I stand behind my advice- check if it's illegal, and push to make it illegal.