It is the nature of a capitalist economy. Supply and demand will drive the prices (high demand, little supply), and there isn't a darn thing PB can do about it. Even if they could, the cost of tracking the scalpers down and punishing them would be prohibitive.
PB must walk a thin line between what old school bluegrass bands people want to hear and what sells tickets. True all the tickets sell out quickly, but without BIG names year to year popularity would dwindle (not saying that's a bad thing necessarily, just saying). Personally, I don't think there is another "promoter" (no offense, just the best word I can think of) that walks that line as well as Planet Bluegrass.
As always, cheers and a hearty WELL DONE to the PB folks!