Going by regular physics, it has to do with the refractive index (or how much light is bent by a transparent medium) of the event horizon. This is what makes a pencil submerged in water look bent. However, the extent of bending depends on the wavelength of the light or electromagnetic wave passing through the medium (which is why prisms can split colors).
The wormhole's event horizon looks like a reflecting pool probably due to the same reason that a mirage looks like one -- it bends visible light. Thanks to Einstein, know for a fact that the gravitational fields around blackholes (and wormholes) bend light. So both the reflective effect and the permeability to radio waves make some scientific sense.
P.S. If you're wondering why a gravitational field strong enough to bend light won't shred the person stepping into it -- that, I believe, is where Ancient technology comes in. They found a way to make the wormhole affect energy only. They transport matter by first converting it to an energy-based data stream, which is reconstructed at the receiving end.