Because sugar and other sweeteners have calories, they regulate sweeteners separately. Artificial sweeteners may not have calories, but they're still lumped in with the sweeteners because that's how people conceive them.
To be honest: the distinction between natural and artificial flavors is kind of bogus. There's no reason to think that the artificial ones are worse for you or taste worse. They're often the same chemicals; the artificial ones are more simple because they contain fewer other things.
The natural and artificial sweeteners are likely to make a much bigger deal because they are nutritive, and have bigger effects on your metabolism.
The term artificial flavor or artificial flavoring means any substance, the function of which is to impart flavor, which is not derived from a spice, fruit or fruit juice, vegetable or vegetable juice, edible yeast, herb, bark, bud, root, leaf or similar plant material, meat, fish, poultry, eggs, dairy products, or fermentation products thereof.
Somehow, "sweet" must not legally be a flavor?
Furthermore, reading your quoted definition closely, it seems to me that "artificial" vanilla made from wood pulp could be considered non-artificial, so why isn't it? Or is it?
I guess someone somewhere convinced people that "sweet" isn't a flavor.
But it's extremely misleading, and I feel like even if I know now to read the label carefully, what's the point if other people don't, so the shelves are taken over by "natural" products with sucralose and crap like that?
Also it seems to me like an ominous sign of the decline and corruption of the regulators.