So does Slingshot.
A tether is cheap insurance. See photo. Get a pair of those zip tie stick-on bases. Peel off the adhesive foam, and replace with the 3M gray adhesive foam that you can get at any auto parts store. Attach to the foil, and run a length of coated kite line from them to the footstrap. Cheap and easy. The coated kite line can be tied and untied repeatedly without fraying.
Windance on Hood River is very insistent on tethers. They have seen enough lost and broken foils to be so insistent.