It depends on the proportion, I guess. Birds do have gizzards which help grind grains to digestible pulp, I think chickens do too,, so eating a little grain is natural. The problem arises when fowl are fed too much on grains, as wild fowl eat things like worms, carrion etc. in the wild as well.
Also, some people come to this diet with such a hypersensitivity to grains that they can't even tolerate any grainfed meats or grainfed eggs.