You have to understand the context here.

This was made in 1970. It was time when many of the old mom and pop diners that has vast or no menus were being bought by chains and switching to a more rigid simple menu.
These places changed from being somewhere you can turn up and just order whatever you want within reason, to having to have exactly as it is on the menu.


These days, the idea of going to a diner, knowing the ingredients they have and just making up an order is completely foreign.

This scene shows how a lot of people felt but didn't want to actually act, because they understand its not how understanding mature adults act.