They do that by staying out of such countries. Many US companies don't want to work with EU GDPR and just block all european IPs, wikipedia has full right to leave UK. They are under no obligations to provide service to them in the same was as pornhub is under no obligation to provide services in eg. a country that would require them to disclose IP addresses of all viewers of gay porn, etc.

Saying that it was a democratic decision without people actually being asked if they want that (referendum) is just weaseling out instead of directly pointing out that it's a bad policy that very few brits actually wanted. Somehow no one uses the same words when eg. trump does something (tarifs, defunding, etc.), no one is talking about democratic decisions of americans then.

Wikipedia has the full right to say "nope, we're not playing that game" and pulling out, even if an actual majority of brits want that.