I think Lisa is right on the money- it isn't just steroids, etc. that makes chicken cheaper, it is economy of scale. According to something I read recently, chickens are estimated to be the most numerous higher life form on the planet! If rodent farms (located in climates where heating was less of an issue) raised rodents by the tens of millions, I bet the cost would go way down!
However, even for a small operation, chickens require fewer resources for more biomass. You can keep chickens in a small shed which costs a lot less to build than extensive rodent racks. You can let chickens mingle in greater numbers with fewer losses, and best of all, you can let them roam outside and find their own food! You don't have to clean cages that way either. Watering, even automatically, is easier.
With these things in mind, you can raise 10 chickens, or 100 chickens, or 1000 chickens, and you will get more biomass with less effort and cost than with mice and rats.
Now feeding whole chickens works fine for snakes that are big enough, though I wouldn't keep them on 100% chicken- I think some variety in their diet is a good thing. I would not recommend feeding chicken parts on a long term basis, and definitely not boneless portions (who would do this anyway, based on cost?).
Chicks/chickens are no more inherently dirty than mice and rats. Rodents can be quite dirty when kept at high densities; so can chicks.
Many snakes that eat rodents also eat birds in the wild. Some examples- rat snakes (includes corns), kingsnakes, gophers/bulls, boa constrictors, burms. I'm sure there are lots of others. I doubt you would hurt anything by feeding them chicks. I have done it in the past when I had access to free chicks and had no problems at all.
I'm really curious- all of these people who wouldn't want to feed chicken to their snakes due to steroids, anti-biotics, etc., what do you eat? Is your diet totally organic? Given that these products are at least relatively safe for human consumption, don't you think any risk to your snakes is pretty slim?