They do this because advertisers won't pay as much for spots between programs, and the networks want to minimize tune-out. If public TV adopts a strategy of clustering commercials between programs, we would be selling a lower-valued commodity to the advertising world. Advertising actually would widen the gap between big and small PTV stations because the vast majority of ad revenue would go to the major markets. Public television stations would become like commercial independent stations. In small markets, there are few independent stations, although network affiliates can survive.
Affiliates get network subsidies for some of their basic operating costs.
These subsidies allow them to chase after the local advertising accounts that make them profitable. Public TV stations in small markets would be like independent stations without any outside subsidy and would struggle even harder to keep their heads above water. Advertisers generally covet younger viewers. It seems unlikely that they would pay a premium for the public TV audience, which skews older. In fact, they can be picked up at bargain rates in the Springfield market.