Perhaps Jeff or RideMan knows for sure why skycoasters are pay attractions, but I would suspect that poor capacity is the culprit. I mean, they can send three people every five minutes on a skycoaster, making a grand capacity of 36 people per hour. If they included it in the price of admission, it could easily be a three to four hour wait. Guests in line are unhappy, and guests in line don't spend money. I suspect that capacity is also why parks charge for rock walls, go karts, and other similar rides. I suspect that the rumored "fourth attraction," if it does materialize and is a pay ride, will also be of poor capacity.
There could be other reasons for a skycoaster charge. I know that the parks have to pay a sort of leasing fee to SkyFun1 (or whatever the skycoaster company is named) instead of buying the attraction outright. Insurance on the ride could also be hell. Someone else probably knows for sure...
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According to the "official" count, next year's new coaster lineup will feature 17 Arrow 4-Ds, 21 TA2Ks, 34 B&M floorless's, 17 Intamin Gigas, 12 beemer flyers, and 247 CCIs. And oh yeah, CP will receive one of all of these.