The purpose of PBS is to take donations and public money and produce free content. I have a hard time believing that there are a lot of people who would buy a PBS DVD box set but decide not to because it was available on the internet -- especially since it's already available for free on the internet in their primary market, just not world-wide. Moreover, if they distribute using BitTorrent, they can include the usual "if you enjoy this program, please support it with a financial contribution" message, in which case the wider the programs are distributed the more people see the message and the more money gets donated.
