Lots of times it’s up to the waitress to assemble and bag the order, making sure the correct dressings and add ons are in the bag. Most restos it’s on the waitperson, not the kitchen. Sad, but true.
If that’s a standard practice, then the restaurants should have compensation built into their wage structure for wait staff who are pulled off serving tables in order to assemble takeout orders. I don’t see how customers could be expected to know.