The "truth" about a food includes whether genetically-modified organisms were involved in producing it.
The truth also includes the benefits of genetic engineering. Maybe we should label non-GE having more mycotoxins? Maybe we should label GE soybeans as resulting in reduced carbon emissions? Furthermore, the truth includes many other thing about the crop. Was it blasted with radiation and useful mutation selected, as commonly happens in wheat? Was it treated with a chemical to double its chromosomes, as is used in many hybrid lines? Was it selected from a mutated bud, a common practice in apples? Was it crossed with a wild relative then back crossed to get desirable genes, a hot topic in tomatoes (whose wild relatives can be toxic). That's the truth too. Why isn't that labeled? And don't you think it is deceptive to single out on of those while ignoring everything else? It makes the thing singled out to appear exceptionally unique because many do not know the genetics of the crops they eat. It is especially so if the thing singled out has a history of fearmongering campaigns being directed at it.
Perhaps those advocating labeling are doing so for reasons that aren't scientifically valid
That makes all the difference. Laws should be determined by reason, not whoever can shout the loudest (where do you stand on cannabis legalization and gay marriage?).
hey, maybe the answer to bad speech is more speech
How about the right to speak when and if you want to? No one is opposing labeling. The issue is mandatory labeling. Two different things. If someone is selling GE corn, or a product containing GE soy, why should they be forced to label their product as such? The choice should be up to them.
why don't the agribusinesses spend their money making the case for food the production of which involves GMOs
Do you honestly think that would work? There are tons of papers published by independent scientists from around the world demonstrating the benefits an safety of GE, and this is very often dismissed by those who oppose GE (and make no mistake, these pro-labeling campaigns are anti-GE) as being part of the corporate conspiracy. There really isn't much the corporations can do at this point with respect to making a their case (not that you should really be listening to what they say anyway). Realistically, if GE food is not labeled, people will say 'Ooh, they don't label it, what are they hiding, it must be bad for you!' and if it is labeled people will say 'Ooh, they have to label it, that must mean it is different somehow, it must be bad for you!' just like when people point to labeling or bans in other countries as evidence that GE crops are dangerous.
It's not as if it's banning GMO-based foods.
No, they're just scaring people about them, thus making them less useful for farmers (whose end goal it to run a business and make a living), which will consequently prevent future advances from genetic engineering, especially for horticultural, minor, and biodiverse crops and small biotech businesses (as well as ones that simply benefit the environment, like Enviropig, which failed because it benefited no one bu the environment and that wasn't worth putting up with the fear mongering), while big companies that sell seed for agronomic crops like Monsanto continue to hold large market shares and likely only lose some sales (after all, did proof that trans fats stop the majority of people from eating them...then again, despite the fact that they are a known danger and GE crops are as close to safe as science can demonstrate, no one has launched any big scare campaigns against the known danger, so I could be wrong about that). I swear, I would not be surprised if one of these days we found out Monsanto was behind this pro-labeling anti-GE nonsense.