My chickens are omnivores. My cattle are herbivores, and my horse is a vacuum. He'll eat it no matter what.
Our cattle are grass fed as grain is very expensive in our neck of the woods. However, I've seen what those feedlots feed, and it's not a grain only diet.
Our grass fed cattle (angus) produce a nice carcass, with suitable marbling. The fat is white, not yellow, and the meat has a great taste. While diet does influence meat production, breeding also has an influence in what ends up on the table. You don't normally see Holstein steaks on the table.
Having said that, the media does influence what people eat. False advertising, outright lies, and so forth. I recommend the consumer research all they can about a product they are buying. Question everything, and let the food producers know the consumers of today are more informed than ever before.