The laws as written above call for a brake that enables the operator to make the braked wheel skid -- a brake is defined as a device for slowing or stopping a moving vehicle, typically by applying pressure to the wheels. A fixed gear is a mechanism that fits this definition and meets the requirements of the written law, so it's actually pretty tough to make a convincing argument that they're not legal. Jamming your foot against the tire or dragging your feet on the ground would not fulfill the law's requirements because the bike is not equipped with feet.