I'm guessing B6 is more expensive because they've tried raising the fares and the planes are still full enough... so perhaps the question is "why would people pay more to fly B6?"

For me flying from SEA, there aren't all that many other nonstops to BOS and JFK, and between the convenience of the TV and the fact that it seems quicker to earn a usable free ticket, I'd probably pay 10-20% extra to fly it...

(now, I priced one itinerary for December 2008, and B6 was $878 roundtrip, compared to AS at $602, which did scare me off.)