We take a cut of all auctions facilitated by our smart contract, but we put no restrictions on owners' ability to transfer or sell their Kitties outside our site. In fact, CryptoKitties is the first smart contract to implement the draft ERC-721 standard, which attempts to make NFTs (non-fungible tokens) as interoperable as ERC-20-compliant fungible tokens. https://github.com/ethereum/EIPs/issues/721

Hey, they're the ones that designed the smart contracts. I think the fact that you can design a marketplace that allows someone to take a cut assures the success of smart contracts since people now have an economic incentive to design marketplaces.