Some places are not restaurants—that is, food is not allowed—and there’s a joke:
 
“A student walks into the classroom, loaded down with sandwiches and soda The teacher yells, ‘What do you think you’re doing? This isn’t a restaurant!’ The student replies, ‘I know, that’s why I brought my own lunch.’”
   
The classroom joke has been cited in print since at least 2008. A taxi version of the joke is from at least 2015.
 
 
Google Books
101 Relaxation Games for Children:
Finding a Little Peace and Quiet in Between
By Allison Bartl and Klaus Puth
Alameda, CA: Hunter House
2008
Pg. 79:
Fred walks into the classroom, loaded down with sandwiches and soda The teacher yells, “What do you think you’re doing? This isn’t a restaurant!” Fred replies, “I know, that’s why I brought my own lunch.”
     
Twitter
mister chloe
‏@chlosephine_
“EATING AT TWO AM? THIS ISN’T A RESTAURANT CHLOE” i know dummy that’s why i’m making the food myself in my underwear holy fuck
12:48 AM - 5 Jul 2011
 
Twitter
TΔSH
‏@tashhrecollet
*eating in class* teacher “this isn’t a restaurant” well that’s why I brought my own food lolol
9:20 AM - 16 Apr 2013
 
funniest-jokes/com (2015)
6) A man enters a taxi with a hot dog.
– Excuse me, this is not a restaurant!
– I know. That’s why I brought my own food!
 
reddit
A man enters a taxi with a hot dog.
submitted July 24, 2016 by DooleyMTV
The driver says “Excuse me, but this is not a restaurant”
The guy replies, “I know. That’s why I brought my own food.”