Because graylist subscriptions will be distributed and managed by peers, they can change over time, and are only relevant when storage contracts are declined. It would be impossible to verify if a given node subscribes to any graylist. A node could hypothetically broadcast what graylists it subscribes to, but given the decentralized nature of the network, there is no guarantee that the broadcasted information would be accurate.
Note: There is no singular graylist. There can be many graylists with different content categories.
At the end of the day, users will be providing a service, and they are free to choose what service they provide. They could follow US law and collect their fees. But perhaps someone in a jurisdiction with more relaxed laws, such as Switzerland, will open up their gateway nodes to any content. Users will be able to control their nodes as they feel more comfortable with.