My LHBS sells only pre-packaged DME that is in vacuum packed bags from LD Carlson; occasionally one will not have a vacuum on it and I don't hesitate buying it. I asked my LHBS owner why he doesn't repackage from bulk and he said that customers have a hang up and don't want to buy it re-packaged. So the buyers drive the sales of more costly pre-packed bags there. If he makes a bit more on the sales, then, hey, who am I to suggest a different approach. He repacks grain in both milled and unmilled, buying Munich, Vienna, Crystals and Roasts in bulk bags and packaging those in 1, 5 and 10 lb bags. If there isn't a grain re-packed when you need it, he will mill it on the spot (in the back of the shop where the mill is located - he wears a mask for his own safety, but I imagine it keeps his snot out of the product, too.) I buy my base grains in full sacks and find plenty of extra items in them as I mill it.
I don't blame anyone for wanting to minimize bad stuff in their ingredients. I have a mouse trap near my mill for that reason.