And you think there’s no acceptable amounts of rat poop in any other food you eat? Funny.
Chickens aren’t grown in sterile level 4 labs. There’s all kinds of contaminants in every food you eat. Many many more so if you buy “organic” because those fertilisers and weed killers are far less pure.
There are exceptions, of course. But you’ll still find miniscule trace amounts of quicksilver in baby food every once in a while, sure. So little that the check machines won’t even register there’s any at all but a lab could still find some. But by and large, there are acceptable levels for everything and we check for all of that, too. There’s a reason it’s an industry, not grabbing a random chicken from the street and biting it’s head off.