If you the business owner want to make a behavior mandatory you explicitly make it mandatory by policy and often by business rules enforced by the computer. I am aware of POS systems for a number of major concerns in retail not in dining mind you where such affairs are systemically enforced without a button to delete fees individually. Adding a adds b removing b removes a. Anyone who is a big enough asshole to charge such a fee after you eat probably is a big enough asshole to enforce compliance from the staff.
You say "most would laugh" and I feel like perhaps you haven't made much of a study of the human species. Most people are wimps and afraid to look like an asshole over a few bucks. This policy is pervasive enough to bitch about because it's damn near trivial to bully people into compliance if it would make them feel or look like an asshole and the cost is small and more importantly if they feel like it's normal and a social faux pas to complain. An authority figure even one as trivial as a waiter or manager telling them that this is normal and customary is sufficient to inspire conformance especially with a room full of people who are paying and leaving and not making a scene.
For myself I would have read the signage and made an informed decision not to eat there on principle but had I sat down and eaten I would in fact pay a modest fee because it's not worth my time to tilt at windmills.
Can you imagine sitting at a table spending your afternoon fighting the battle of the sandwich fee with a "manager" himself paid a pittance and counting yourself the victor?
Winning looks like walking away and knowing they are slowly alienating their customers and they will be dead next year.