Remember, the Winklevii are trying to set up an ETF so they can dump their Bitcoins. They need to find enough suckers.
I don't think hey are trying to dump their bitcoin. I think they are trying to profit off of bitcoin. They will obviously sell a portion of their bitcoin when the ETF starts trading, but I think they hope to be able to make most of their money from the fee that the ETF charges it's holders (the management fee). I would also not be surprised if a fee what charged anytime that an AP exchanges the ETF for BTC or BTC for the ETF.