Surprisingly, many chickens (Cx-types obviously excluded) seem to know (somewhat) better than that. Just as many of us who can afford to do so don't eat ourselves into obesity (but some do, just as with chickens).The biggest difference lies in the style of feeding horses vs chickens. If you gave most any animal free choice big bins of various foods to choose from, a large portion of them would most likely eat themselves to obesity and death, just as many humans do. And we supposedly "know better" too.
The bigger leap comes in comparing a study with a number of different rations/ingredients available, including concentrated animal protein sources, and assuming pasture raised/free range is directly analagous. Maybe it is, maybe it isn't - depends on the pasture and the season.
Obviously, I'm not against free range/pasturing - I do it myself, and I continue efforts to improve my pasture - but I also provide a high quality nutritionally complete mix as part of my risk management. Those who want to assume their pasture is adequate are engaged in far more risky behaviors than I am comfortable with myself.