They still have to abide by SAG and Writers Guild contracts, they still have to clear stills, and they still have to deal constantly with music rights and existing distribution agreements. PBS also show stacks of shows acquired from elsewhere or which are international co-productions.

The answer is they'd have to pay stacks of people for streaming rights, on a tiny budget. I really pity the poor bastards at PBS who had to try and arrange the underlying copyright clearance for this YouTube deal, which must have been an unholy nightmare given their budget.