This essentially "protects" Big Pharma. Under the Dietary Supplement Health and Education Act (DSHEA) vitamin C is a dietary supplement, and as such, it can only make structure/function claims (i.e. Vitamin C boosts the immune system). DSHEA came about after intense lobbying against the FDA's attempts to control supplements, herbs, etc. and restricts its authority over them PROVIDED manufacturers make no claims about their products treating, preventing or curing diseases, etc.
Making such claims about any product makes it a drug - and since Big Pharma is the drug cartel, only Big Pharma can make those claims. This is why I'm not allowed to tell you that lavender helps insomnia, muscle pain, and acne, or that it helps heal burns, bites and abrasions - even though we all know it does all this.
This is also why we don't claim e-cigs help smokers avoid tobacco cigarettes. To do so would be claiming that they treat a disease, which means they're drug, which means they belong to the FDA and BP.
So basically it's okay to say, "Vitamin C boosts the immune system." not "Vitamin C prevents scurvy"