You can use folding knives for food prep. In fact, I'm sure quite a few members here do it regularly. Food could potentially get inside the pivot and handles, but that typically doesn't happen unless you're really sloppy about it. A design like the Kershaw Tilt would help prevent that, as the edge itself sticks down and away from the handles.
Of course, most kitchen knives are ground thinner than most folders, so it might be more difficult to cut with one. But I don't find that my BM 710-1 is more difficult to cut with than say, my Tojiro Flash paring knife, so there might be some exceptions.