Well I think it's a little more complex with things like wheat.
Sure people with celiac disease have to abstain from wheat, but for most people it's fine. BUT that's only in the appropriate balanced amounts. Most people it wheat 3 times a day or more! And if they have been eating a crap diet for years and their organs and gut have strained which is going to be further worsened by eating to much things like wheat.
It's sorta like salt, eating too much salt is bad, but it's also bad if you don't get enough salt! Now wheat is not itself an essential food, but I think people become phobic of certain foods after they abused them by overeating that they go the other extreme and either eat to little of them or completely eliminate them for good.
Yes sometimes health conditions require you to eliminate some foods to heal the from condition cause by overeating those same foods, but it's only temporary until your healed from it and your body is back in balance.