So here is the deal, every single plant we consume is gmo, even if it says non-gmo. Why? Because every plant we grow has been selectively bred, cross pollinated, spliced, etc, until the plant is the way we want it. This process occurred long before laboratories were a thing. There are some people who claim even that much change is unhealthy for us, in which case we're pretty much boned because that would make all food unsafe except wild varieties that have never ever been cultivated at any point in human history. The only difference between oldschool gmo's and and current gmo's, is the way in which the genes are influenced. That's about it.
When it comes down to it, it's not a matter of all gmo's being bad, or all gmo's being good. It should be evaluated on a case by case basis. If they modify a grain to have a thicker husk so the bugs can't eat it, that might not produce a negative effect in a person, but the grains that produce their own pesticide to repel bugs, might be harmful. A specific study showing what specific chemical chains are different between the two, and what effects those have on the diet of people who consume them are the only way to tell if they are harmful.
Putting gmo or non-gmo on a label is in and of itself highly misleading because all our crops are genetically modified, gmo only refers to ones done in a lab, using a more advanced method, and telling you it is gmo or non-gmo doesn't in any way tell you how safe it is for you. Something listed as non-gmo could easily be worse for you than a gmo item, and a gmo item could easily be worse for you than a non-gmo item, again it's case by case.
I have yet to met one person who is on the non-gmo band wagon that can explain why, that can cite even one example of a particular gmo crop that now contains a protein or carbohydrate chain it didn't used to have, and why that is in some way worse for you. They wouldn't even be able to tell you if there are particular gmo crops that they don't think are bad, and ones they think should be avoided, they just think non-gmo = good, and gmo = bad.
Companies stand to lose a lot of money if they're forced to put a label on their products that the ignorance masses have come to associate with evil for no valid reason, and as I've already said, the term is in and of itself not a very clear or useful designation. How was it genetically modified, is the result of the modification in any way bad for a person? That is more information than a food label is usually going to contain. Do most people realize the thousands of actual chemicals chains that make up the food we eat, and that it's all boiled down to a hand full of categories on a nutrition facts sheet? Probably not. A company only has to list protein as a whole category, even though the specific proteins and their amino acid contents is extremely important to nutrition. Most food labels only list a handful of fat types, even though there are more. Most food labels only list total carbohydrates, dietary fiber(soluble and insoluble) and sugars, even though there are thousands of overall carbohydrates, and dozens of even what we would consider sugar. Is it glucose? fructose? sucrose? galactose? lactose? maltose? lactose? Is it any one of many oligosaccharides? The same pretty much goes for fats, below their sub categories there are thousands. If you approach the fatty acids as a group, below the omega groupings there are usually several groupings below that. There are 3 types of omega-3's for instance.
In general I think most people are so ignorant of nutrition and biological chemistry that adding something as generic and obscure as gmo or non-gmo to food labels does nothing for the average consumer. I kind of chuckle to myself when i see fat free on a pretzel bag. They didn't put fat free on pretzel bags until fat free became a catch phrase in the food industry, even though pretzels were always fat free, and eventually studies have shown fat isn't the boogie man it was once thought to be, but that is how susceptible people are to manipulation. As far as I'm concerned the whole campaign is an effort to drive up sales of products labeled non-gmo, which the "organic natural cage-free" market has turned into a juggernaut being both high profit and a growing segment of the market, they actually have more clout than some would imagine, when compared to "big agriculture" or however people choose to imagine corporate conglomerates.
It reminds me a bit of something i was reading recently about chiropractors. The reason most insurance companies allow chiropractor visits at all, is that chiropractors as a group lobby to make it mandatory for insurance companies to allow people the alternative. Would most people imagine chiropractors being influential lobbyists in a world as high dollar as the world of medicine? Meanwhile you have chiropractors waving crystals over people, doing colonic irrigation treatments, and other similarly hoaky things, and insurance companies have to pay for a lot of it, whether they want to or not. Most people would imagine the chiropractors as being the alternative to big medicine, not themselves being "big homeopathic".