As someone in the Eastern US (grain of salt there), this rarely occurs but if it does it is usually when a place serves a large group (justified as the extra difficulty for serving so many people at once). If I saw this applied in another situation I would 100% consider that a tip, give them nothing, and never eat there again. If it was applied when I went in a large group I'd say it can go both ways, but I'd definitely not go back there with lots of people again because it feels like its an attempt at fleecing the customer for more than the trouble is actually worth. Alternatively I might tip less, taking the 18% into account already as a tip. Overall, I'd say it's bullshit and a good proportion of the people I know would agree.
Hope your trip goes well
Praise be to GURPs! It's unfortunate that there seems to be a persistent sentiment that DMs should be making snap arbitration on a large variety of systems instead of having a rule-base that you can ignore when it gets in the way of your storytelling.
GURPS does this some much better because it does have rules for almost any genre and style you want, letting you have professionally crafted rules that have been playtested and matches to the genre they are designed for that you can use either way.