There is almost no whole-grain bread on the market in the US that does not have added sugar.
AIUI this is because the whole wheat flour tends to impart a bitter taste, so the sugar balances it. The sugar also enables the yeast to raise the bread faster, helping mass production.
Interesting, the only mass-produced whole-grain bread I have on hand is Ezekiel and it didn't list any added sugar, or indeed any sugar at all. It's also not listed as an ingredient on the (partially) whole-grain bread I got from our local bakery.
Having said that, I see your point that setting too low a threshold might be unnecessarily onerous on cafeterias and manufacturers. Perhaps a threshold of ~1g sugar per slice would be more reasonable?
Ezekiel bread is one exception, but they are also (at my local big box store) 3x the price (by kcal) of the white-label whole wheat bread AND less shelf-stable (thus will incur more process losses and expenses).
Interesting. Near me Ezekiel is a little less than 2x more than the cheapest bread-like sponge loaf but only about 50% more than the cheapest whole wheat bread that doesn't dissolve if you spread peanut butter on it.
That's not nothing of course, especially for a school trying to minimize food costs. Many schools (like the one I attended growing up) procure the cheapest possible USDA/NSLP-approved product.
But kids at these sorts of schools seem like they'd see the greatest benefit from this sort of legislation to raise the threshold of what constitutes a healthy lunch.
AIUI this is because the whole wheat flour tends to impart a bitter taste, so the sugar balances it. The sugar also enables the yeast to raise the bread faster, helping mass production.