You're showing yourself to believe that America can regulate conduct that never touches American soil.
If you enter America, there may also be consequences, but you don't need to enter America.
You picked a bad example - there are many US crimes that you could get away with if done elsewhere within the local laws, it generally isn't seen as worth bothering with when done elsewhere if the other country doesn't care.
That isn't much different. Say an adult American drinks alcohol in America; then they travel to a country where alcohol is illegal. Should they be prosecuted in that country for having drank in America?
There's a world of difference here. Ofcom is claiming to be able to shut down an American website for content generated in America, stored in America, and shown only to Americans. There are no UK citizens in this chain at all. This sets up Ofcom as having global censorship authority even over content seen elsewhere.
> Should they be prosecuted in that country for having drank in America?
In my opinion, no, but some countries are hardasses about this. If you want to do things that are illegal in certain places, you should not plan on traveling to those places. Usually, they will just refuse you entry but you kind of do put yourself at their mercy if you touch their soil. This is how the world works.