It isn't the parts of the animal but the fact that it is washed in ammonia and maybe subjected to other indignities. Yuk., Bye the way, I read that when meat starts to go bad in the supermarket it is often soaked in bleach. I once bought a piece of mahi mahi for my daughter and on the package there was a picture of a lemon and the words, lemon fresh. When I got it home and opened the package it was obviously spoiled and the market had used the lemon juice to try to mask the odor.
I also always tear the plastic wrap on boxes of mushrooms and sniff them to make sure they are fresh. And often also tear bagged potatoes. I have been stung too many times.
I have also noticed that even the fruits and vegetables have a better color and look fresher in the super market, no doubt due to the lights. That nice red tomato always looks a lot duller when I unpack it at home and the dark, shiny purple eggplant somehow loses its luster between the market and my home.
Didi in a lot of small, independent butcher shops diluted bleach is still widely used on meat and also in some Chinese restaurants I've been to. I've sent bleached food back in one place never to return. Old meat gets slimy and the bleach takes it right off.
In the big chain stores like Safeway, it doesn't happen anymore. Well not around where I live anyway. A store could get closed down for something like that.
Years ago I read some books written by Adelle Davis, the famous nutritionist in the 1950's and she suggested bleaching all meats for bacteria control. I did that myself for awhile in my carnivore days and the meat lasted a lot longer, then stopped because it just didn't feel right.