Legal, but useless
Read the numbers carefully. They are talking about generating microwatts of power at only millivolt potentials. The Volt's 150 HP (112 kilowatt) electric drive system would require nearly 19 MILLION 6mW antennae. Good luck trying to pull all of those on a trailer behind your Volt.
All joking aside (well...I hope you were joking), the practical use for this sort of power source is microelectronics, particularly remote or wireless sensors. Instead of stringing miles of copper cables throughout a skyscraper or bridge, engineers are currently developing wireless sensor networks to monitor the temperature and force stresses on the structure. You unfortunately still have to periodically send someone around to change the batteries. If they could power themselves via the airwaves, they would be virtually maintenance-free.
