That's not a fair comparison, unless the restaurant also brews its own beer.
A better drinks comparison would be choosing from the menu between a 'Jose Cuervo Authentic Margarita' and a margarita mixed fresh by the bartender.
I agree with the OP - in a restaurant I would always order a made-from-scratch dish (the house margarita) than a factory-made burger (the premixed margarita).
Actually drinks were the first thing that came to my mind as well. With drinks it is basically normal to order by brand (several brands of beer on tap for example). But there are also restaurants and bars that brew their own beer, and people go there for exactly that purpose. So it is not directly comparable, maybe, but at least comparable and a good example on how you actually think about this with other parts of your diet...
A better drinks comparison would be choosing from the menu between a 'Jose Cuervo Authentic Margarita' and a margarita mixed fresh by the bartender.
I agree with the OP - in a restaurant I would always order a made-from-scratch dish (the house margarita) than a factory-made burger (the premixed margarita).