The with a prepackaged food the chef has no influence on the food. And so neither the patrons of the restaurant. Having the choice to bar the prepackaged food gives consumers better control on many aspects of the food. Directly or indirectly.
Even for the foods that are nearly completely ready to server the ingredients, recipes, and flavor profiles may have been created by the actual chef from the restaurant. The packaging process offloads the labor in this case but not the creativity. In that case the argument against prepackaged food seems to be against time/place shifting labor more than anything else.