Most commonly it's the EU fining American tech for GDPR violations and related privacy shenanigans.
If Imgur never had UK presence, then yeah there would be no teeth. But if you're doing business in a country you can't break the law then leave and expect them to just ignore what you did during that time.
How do you expect the "pull out" to happen? They must have had a UK bank account or similar, whose transfers won't get approved as they're trying to escape from criminal prosecution. Or they'll work with the US to ensure responsible individuals are held responsible.
It isn't exactly the first time someone/something commits crime in a country then try to escape, there is lots of ways to work with others on this.
May US voters put America First over international law.