Really, it's that there is a menu, but they won't show it to you, only tell you when you ask for something they won't serve.

"What would you like to eat today?"

"A bacon cheeseburger with cheddar, with lettuce, ketchup, and onions. Fries on the side."

"Sorry we don't offer fries with our burgers. What could I get you instead?"

"I see french fries at the next table!"

"Oh, you asked for fries, not french fries. Sure, we can do french fries. What would you like for your side?"

"... I want french fries."

...

It's not actually true that any input is accepted. Pretending that it is just creates frustration.