I can't see how it's legal to impound the aircraft.
As I understand it, they can only start a civil action for trespass, and attempt to show that the pilot did some damage for which they should be compensated.
In the meantime, however, they could, if they were sufficiently bloody minded, (and it appears that they are) refuse to allow the owner the use of the runway for takeoff, leaving him/her with no option but to remove the aircraft by some other means.
MJ
