Said companies often find it less burdensome to comply than the option of being outright blocked from the market. Brazil did that a couple times with a couple different companies. If a company wants to provide services to a given jurisdiction, it needs to comply with local regulations.
4chan offers services at it's web servers which are in the US. People from the UK come over, access the servers in the US, and then import the content they see into the UK. 4chan is offering services locally in the US. People come from all over the place to access those services in the US.
4chan isn’t in the UK. 4chan doesn’t have UK employees or offices. 4chan doesn’t book income on the UK. 4chan didn’t have any thing to do with the UK at all.
If this isn’t convincing, consider this: legally what is the difference from Afghanistan requesting anything not legally in compliance with Taliban’s laws be restricted? Would you support that? Legally that is what is going on here.
Afghanistan can block anything they want within their borders and hold anyone in contempt if they refuse to comply. Whether I support that or not is immaterial - I would have to comply in order to continue making other content or services available to Afghanistan residents. Being entirely blocked would be worse.