Those people just won't get even close to the same nutritional value out of it as grain of higher quality that would be wasted (and unprofitable) as livestock feed. The quality of soybeans, wheat, and corn used in livestock feed is nowhere near the quality of soy, wheat, and corn used for people, FDA guidelines aside. It has more to do with soil and climate, and the protein they can impart into the grain, than FDA guidelines -- this is what grain farmers have told me with regard to who they can sell to, and get the highest price for, their crops.
As I said about quality, the significance of that varies by the nature of your customer.
Animal feed grains do indeed require protein supplementation for healthy feeding to all livestock. Thats why protein components that are often not human-edible are mixed in. And so are human edible ones too. But that does mean that protein content is indeed a desirable feature of feed grains.
If a piece of land is not as suitable for, say, wheat, and is best used for soybeans of lesser quality according to market conditions, it does not mean that the land cannot be farmed for lower yields of wheat or for another crop entirely. If a piece of land cannot be farmed or profitably used in agriculture for anything but animal feed (which seems very unlikely to me), and if there is some limitation on animal feeds that reduce the market for them, then there would indeed be some environmental benefit to letting it lie fallow or revert to wilderness, though this benefit is probably bought at a foolish price.
And then there are the inputs that go into growing this stuff in the first place - fertilizer, water, fuel, equipment and labor - that is less available for growing human-edible crops, all of which are more limited than viable agricultural land.
I am absolutely not for banning or limiting beef consumption; beef is a good thing indeed. I am also absolutely not for government controls on these things either.
But facts are facts, and if one is to argue with the other side it is best to be scrupulously accurate. We will always win with the facts.