That’s wrong of course.
A person from the UK with a site in the UK, have the laws of UK and not US laws. If it is wrong to gamble in the US, it isn’t in UK. So the site respects the law. That person does not have to block access from anyone. People from US are responsible for their acts, and that people must respect their laws. The only entity that can take any steps into blocking anything is the governement of USA. That entity can order an ISP, within USA to block access to foreign websites.
Do you think I have to compile a list of 300 countries in the world, study their own laws and block access as needed, or do you think I only have to do that for USA? Again, with all the respect, USA is not the world, and US laws apply to US.
And if online gambling is ilegal in the USA, it means that for a US citizen it would be ilegal to advertise it even if blocking access from US citizens. Otherwise would be like saying selling drug to Americans is ilegal, but because it isn’t at country X, I can export it.
And, also, censorship is ilegal at my country, so I can’t block access to a website.