My thoughts on this are that unlike humans who have the ability to choose to be a vegetarian or not, chickens are pure instinct and their instinct is to eat anything they can find/catch. The day I see a chicken not get super excited about finding an earthworm and gobbling it up, is the day I will say..."that chicken chooses to be a vegetarian!"
I don't agree with vegetarian chicken feeding because of the above but as already stated on this thread, maybe not a big deal for chickens raised as meat birds due to the short life cycle. On the other hand, I know I get cravings for a big ol' steak or hamburger and would imagine chickens deprived of non-vegetable protein might crave the same way given that they are natural omnivores. I believe all animals instinctly know what they need in their diet.
I always have issues with humans imposing their lifestyle on animals or forcing an unnatural diet on them to save money. In my opinion, it has a level of cruelty to it that seems unnecessary. As we become more and more aware of what is in our food, I prefer animal protein in which the animal was allowed to eat what it's supposed to eat, not what humans decided it should eat. For that reason, I personally would not buy a "vegetarian" chicken to eat.