some resturant really have such rules - no outside food is allowed - as they don't want to take any responsibility if anything got wrong (eg food poisioning) they did not want to be blamed.
usually - people will ask if tha't o.k. to bring outside food to the restuarant.
for us, we did try one of the "all-you-can-eat" in markham. you will have to tell the waitress first that the kid (only yrs old) will not take any food from the restuarant, otherwise, they will charge you one headcount. so always check before you go in.