It's mostly in the way chickens are processed in the US, dragged
through vats of liquid in which the bacteria have accidentally been
introduced. The length of immersion has been said to be long
enough so that pathogens can contaminate not only the surface
but also to some degree the interior of the flesh - different from
other meats, which are slaughtered and processed mostly dry, with
any washing being done using running water spray rather than
immersion.
Seems to me that if chicken bathing were done in a sufficiently
concentrated brine, this issue could be obviated.
In any case, I'm comfortable eating rare or even raw chicken in
other first-world countries and eating pink-in-the-middle chicken
anywhere in the world.