Because like so much more, in small doses it's fine. Unlike the previous answer though, I think you may be on to something when you say
I understand that the dosage here, if any, might not be lethal; am
more concerned about trace amounts causing inefficiencies in bodily
I think the only concern being about the conversion to cyanide is a little misled though.
See here for the first case of poisoning by potassium ferrocyanide. The guy drank two glasses of it of course, so what's in your salt will never kill you.
Please don't let me speak to any health implications though, I'm not a chemist or biologist. Just providing a source I found and adding my own two cents.
For why it's allowed, there's no evidence it hurts people, and certainly isn't lethal when used as allowed. whether or not it causes slight inefficiencies in bodily functions... I think it's likely it does. But I have no basis for saying that, and it certainly won't stop me from eating table salt with potassium ferrocyanide. Again, fully an assumption, but I assume the 'harm' is negligible. Also, peppers can be poisonous in large doses and have a host of health benefits when eaten regularly and in small amounts. I think we need some serious funding and studies on it's effect on the human body over a real sample of the population to give a definitive answer.
TL;DR; I don't really know, but it's clearly poisonous in large doses, so as far as I can see small doses have to have some, maybe negative, effect.