It's not the dextrose that's at issue, it's the culturing of cultured dextrose. According to wikipedia
Cultured dextrose is a food additive used to inhibit the growth of undesirable bacteria and mold in food. Often used in place of benzoates and sorbates, it is considered by some consumers to be a more "natural" ingredient, because it is prepared by the fermentation of milk or sugar powders...
Cultured dextrose is marketed under the tradename MicroGARD® by Danisco, a unit of DuPont, and is used in a range of foods including dairy products, salad dressings, and baked goods.
When you go to Danisco's own website
, you see a list of products under the trademark MicroGARD®, half of which are cultured using skim milk. I have no idea how this stuff is listed on packages (because I've never knowingly come across it) and if it's indicated whether the variety used in a product is one of the ones derived from skim milk or not, but if not, it's something I'd be wary of.