They are getting protein in their grasses too. (Consider grass fed beef cattle) Now you probably won't speed a hen through molting on grass alone. but it's in there. And among the grass blades as you know is the animal protein (insects, frogs) to compliment the grass. Free ranging is set up for a complete diet for your chicken, in my opinion. As fall wears on to winter, I notice my birds are ranging farther and farther from my yard to find what they need. Sort of amazing if you look at it from afar...but then I get nervous of course and more neighborly if you will and scuttle them back home with enticing offerings. LOL.
Just this fall with molt and daylight hours shortening-egg supply not keeping to my needs I bought some Amish eggs from my grocery store. This guy has his hens raised on hay, outside birds to some degree, but I can tell they are being fed mostly bagged feed. The yolks are much paler than my grass fed free ranging girls. Grass = more betacarotene in the egg and perhaps the more delicious flavor.
Cows and chickens are completely different. The protein in grass is low. Of course there are bugs & critters in grass. Are there enough? Probably not reliably enough. And probably not year round. And certainly not if there are very many birds competing for it. Domestic chickens are not the same as wild fowel ...
Though some people can get away with not feeding their free-range flocks and have gorgeous healthy birds, I personally think it is irresponsible to recommend that as the only right way to keep chickens. Especially in backyards where the range area is limited in size and diversity.
I don't believe it is wrong to feed your birds.