I'm sorry you're so cynical about the feed industry. The label in fact does equate to what is in the feed, there are state and national laws regarding what and how must be listed on a feed tag.
As to the assertion that companies simply relabel products that don't sell, well that is an out...
Corn is not a filler, it is a source of carbohydrates (energy) in livestock rations. All this talk of lack of nutrition, being a filler, empty calories is simply not true.
However, corn is NOT a complete feed. The Amino Acid level and ratios are not appropriate for chickens to thrive.
Corn doesn't cause the build up, the improper nutrition causes the build up.
We are picking nits and on the same side of this argument. Corn isn't bad, feeding it improperly is.
The corn in and of itself is not the problem rather is it the imbalance of nutrients when you feed corn in addition to a feed. When you do so you no longer have the appropriate balance of amino acids, vitamins, and minerals and an excess of carbohydrates.