This just makes my jaw drop.
First, I don't think the designer can enforce this...and if it bothers them so much that they can't control the end use, maybe they shouldn't be designing for the retail market.
Second, there just isn't that much money to be made by doing this. Chances are a quilter or crafter (on this board, at least), is buying fabric at retail and just barely making enough on the craft item to make it worth their time and energy. The really big money comes if someone is making thousands of an item and buying the fabric at a discount. At that point, the manufacturer probably knows who they are because they're going directly to them for the discount.
Janet