Today, musicians make products (music) that can be easily duplicated at no cost. We acknowledge their value by compensating them through the purchase via proper channels (CDs, iTunes, concerts) and express our appreciation by being fans.

Software creators are not unlike these musicians; you pretty much need to have a record deal in order to get the money and the mainstream fans.

In Zed Shaw's case, he's basically using GPL ensures that other people are not using his work to get the record deals.