and some more
<http://www.newscientist.com/news/news.jsp?id=ns99992461
"While normal wood is transparent to radio waves, Oka's blocks them because it contains fine particles of a magnetic material called nickel-zinc ferrite. When an electromagnetic wave hits the ferrite particles, the magnetic part of the wave is absorbed."
So the magnets absorb the magnetic component of the radio wave which is what I said.