A little research into flour sacks will also show that the sacks originally used were printed. In the depression, companies sold goods (flour, sugar, animal feed, etc.) in fabric sacks. As added incentive to buy, the companies used fabrics that the housewife/mother would use for clothing, curtains, quilts. It is quite interesting to see the variety of items made from the old sacks. A friend of my mother's told of wearing underclothing that still had the brand name of the sugar company on it.
As to current flour sacks, there are still companies that use the fabric. We pay a premium for it here, to get flour in fabric. I have not used the flour sack (towels?) from Sam's. Let us know how it works.