Potassium sorbate - aside from being a naturally occurring salt - is commonly used for sauces like ketchup and mayonnaise. What a surprise.
Apparently it was a surprise to you, because you thought there were no preservatives in it.
If you want to change your argument from the lost case that they contain no preservatives to saying that they contain no harmful preservatives, then you might still be wrong.
First of all, it being a naturally occurring salt means nothing. Cyanide occurs in nature. And sugar —albeit healthy in it's natural form— is a common junkfood when refined that's known to damage the liver among other things. Yet, it's still a naturally occurring substance. Some refined sugars are even labelled raw.
Second, Potassium sorbate has been linked to many health ailments. Just look it up on scholar.google.com. Not all studies are conclusive, but they still indicate that potassium sorbate is a likely condidate for causing many ailments. So it's a risk to consume it and thus people who's health is important should avoid it.
Also, McDonald's hamburgers are a chemistry project. There's lot of things in there that are likely to be damaging to health. So by eating that one "food" you're actually taking many risks at once. Want to gamble that all those studies linking many of those ingredients to disease are misleading? That's fine, maybe you get lucky. It's your health that you'd be gambling with so it's your choice. But you are gambling with your health.
look it up, you'll be surprised.
I was not surprised. I'm not sure why you thought I would. I know that vitamins get chemical names; they are chemicals. Even water is a chemical. Was that supposed to surprise me?
Maybe you should look it up and be surprised. Ascorbic acid has been linked to kidney stones. It's controversial and thus could very well be fallacious, but I do not have the resources to personally verify the claims so from my perspective as consumer it's a risk to consume it. A relatively small risk compared to many of the other stuff that a McDonald's hamburger contains, thus I may consume it sometimes, but still a larger risk than most foods I typically consume and thus if I can easily avoid eating it then I will.