Why do we put vitamin C on the label? Why sugar content? Why not carbon atoms? Why anything? The FDA (supposedly) requires labels for things that are considered important by health professionals and consumers. If enough people want to know about GMOs, then it's important. Not labeling GMOs or any other ingredients or by-products that might be seen as "gross" to the consumer, is only about protecting the food manufacturers.
I think it's a BS argument anyway because the average consumer, given two bags of apples that look identical, is going to buy the one that is cheaper. And if for some reason the public decided to never touch GMOs, then so be it. Food manufacturers can make their products in a way that consumers want to buy them. That's a free market. Not being shady and hiding information.