It's blatantly obvious. As we see this does happen even in the presence of regulations due to secrecy and denial of investigation. What could possibly incline them to stop this business if no one were investigating them? In my link you can also see how this industry continually grows. It's ordinary logic, what would happen if none or less regulatory laws were made, and no one or fewer were there to prosecute? Do you really believe that it is possible that this illegal business would stagnate or decline, when it is growing despite the presence of regulations?
As to the comment that the consumers are the best regulators, I would say this is pure imagination. Most consumers does not have the technical nor the medicinal knowledge to know what is healthy and unhealthy ingredients in products, especially the synthetic ones. And what then if it was not illegal to refuse to put all the essential but subtle ingredients in the description of some product? How then could you rely on the consumers judgment? It's not like "if it tastes good, it's all right, and if it tastes bad, I'll stay away." Taste is the easiest thing to manipulate chemically.