On the radio interview: I thought David sounded unfair, espousing some unlikely conspiracy theories suggesting that the IWF chose Wikipedia for any other reason than the fact that some disgruntled Wikipedian submitted it to their tip box a few days ago.
The idea that they've blocked Wikipedia but not Amazon because Amazon has more lawyers sounds pretty plausible to me (what else could they mean by "pragmatic"?). I thought the bit about it being an experiment sounded rather unlikely, I think it's far more likely that they just didn't think about what they were doing and have now got themselves stuck in a corner.