If it affects UK citizens, living in the UK, then there's jurisdiction. Either the entities comply, remove their services to the UK, or they risk sanctions/being arrested when abroad/etc.

Why should a US company harm UK citizens just because they're in the US?

If you want to serve a market in another country you have to follow their rules.

In this case, Imgur have been misusing UK children's information. Considering the laws are pretty similar, I suspect they're misusing EU children's information too.