Most of accept seemingly 'odd' Customer Service relationships at restaurants.
Do you leave a gratuity at any place that does not handle your prepared food?
For buffets - we leave a gratuity, even though we gather most of our own food.
How many folks leave the same $amount for a can of beer as they do for a detailed cocktail or soda?
I have been to quick service places with working Service-personnel walking in the dining room all my life. It was customary to leave a gratuity for the people who deliver our food and who clean up after us. More plainly - we used to leave a few quarter$ on the tray for the person who carried away our trash and dirty dishes - they kept a glass or jar with all the change. Quarters turned into $dollars. Fast Food changed that - we started to throw away our own trash....and leave our mess.
If there are people in the dining room at Polite Pig who can bring me sauces, napkins, water refills, cream & sugar, maybe even a cocktail refill, and how about handling food complaints - I'm willing to leave a token of my appreciation. I'm more willing to do so when I leave my trash behind, and all the more if my child has left behind a mess inversely proportional her age that the next family would like cleaned before being seated.
Personally, I'd rather a large tip jar where the $5-$10 from every tray was going, but some folks use Magic Bands & credit cards and that happens at payment.
Folks can always leave nothing at the Cashier and put a few $dollars of appreciation along side their trash - like we do at many restaurants - when they have finished eating.