"But you should not be free to sell any kind of fat you want."

No you should free to sell any fat, but one must not commit a form of fraud by not disclosing what kind of fat you are selling.
If someone sell motor oil and the buyer is led to believe it's olive oil, it's a problem.

I'm not against a Chinese restaurant using MSG, but I think that since it can have a bad reaction for some if not all people, they should required to notify that they using MSG- and if they offer not to put MSG on the food- that they actually don't put MSG on the food.
Same goes for dairy products, or peanuts, or whatever- one should have transparency about what one buys.

"I thought Libertarians felt drug addicts should not be treated like criminals, but that drug peddlers should still go to prison. Or does that too, fall into your definition of free enterprise?"

Doesn't sound like a libertarian to me.

"You keep telling me how you're all about free markets, and reasonable regulation, yet nothing that the business does, no matter how vile, seems to bother you. I don't think the words mean what you think they do."

I think the major aspect of regulation should be about a consumer knowing what they are buying. So, meat which rotten should not be sold as fresh and safe to eat.
But I have no problem if the buyer offered meat which rotten and long as customer knows it's rotten. Now, why anyone would want to buy rotten meat, may be unusual and perhaps potential for fraud, because that person may turn around and sell or give that meat to someone who doesn't know the meat is rotten- but other than that I don't see a problem with it.