A practical joke?
One person in the Bad Astronomy thread on this subject said it is
possible that the whole thing was a joke just to see if the police
would send out a squad car to investigate.
If that is the case, the perpetrators must really be loving it that the
hoax made the BBC news and people are discussing it on the net.
Because the thought that someone who has spent their entire life
on this planet doesn't recognize the Moon even if they know nothing
else about the Universe is just too mindbogglingly depressing to
contemplate.