It's bad optics to build their own Hadrian's Firewall, so they are trying to bully foreign companies into compliance instead. If they want to go after the ad revenue, they would have to try to identify and prosecute the UK-based companies doing business with 4chan, and they will struggle to do that when they have no ability to subpoena 4chan for their business records.
Such firewall exists everywhere, because courts can block access to various websites on different grounds (malware, copyright infringement etc) everywhere.
