My guess it that PBS still needs to control distribution of their content because it is still a valuable source of fund-raising revenue. Revenue from DVD box sets [1] and international licensing deals goes back into producing new shows. Setting their content free on BitTorrent would quickly erode the revenue generating potential of that content.
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.
[1] http://www.shoppbs.org/home/index.jsp