feeding corn to chickens is a cheap option for the chicken factory. I believe the US still have a lot of excess corn sitting around (mostly because they still subsidize corn farmers).

As the product you saw said 'free range', which means its out in the sunshine, able to have a wander about and is in its semi natural environment. In feeding corn (which is essentially a vegetarian diet) to chickens this results in the colour of both the flesh and the skin to be a yellow hue and its meat will be plumper.

whether this is a healthier option.. i dunno.. i think the best package labelling to look for is pastured or pasture-raised. Pastured suggests that the birds lived on pasture and got some of their food from the pasture environment. For poultry, this usually means that the bird get about 20% of its food from pasture source (grass, seeds and bugs) and 80% from grain/grasses feed mixes (corn, oats, soybeans alfalfa, clover, etc.).