Not excusing the amount of sugar they use in the buns, but yeast raised dough needs to have at least a little sugar (or honey, maple syrup - something with a sugar component in it) in order to feed the yeast, so the dough will rise properly. It doesn't need to have enough sugar to make the finished product taste sweet though, just enough to feed the yeast, so they could certainly use a lot less sugar in the bun recipe.

Even a LC yeast raised bread recipe I have calls for 1 tsp of honey or sugar per loaf, in order to feed the yeast. The creator of that particular recipe insists that every bit of the sugars in that 1 tsp will be consumed by the yeast, but I sometimes have my doubts about the yeast consuming every last bit of it. Still, even if there's a half of the sugars left over that aren't consumed by the yeast, whatever is left would be divided between 16 slices, and unless you're eating a lot of the bread at one time, makes it rather negligible.

Commercially made bread products really don't need as much sugar in them as they have - they only use so much to make the bread taste sweeter. There's a reason that visitors to the US from some countries complain that they can't get real bread in the US, that all we have is cake. (I've heard this specific complaint from Australians, but visitors from many other countries probably feel the same way)