(I am not a lawyer. This is not legal advice.)
Gambling sites are an interesting choice for making this point, because even for prominent cases like like FTP it is hardly a matter of international agreement. In fact, Antigua and Barbuda have previously successfully beaten the US in WTO courts over this very matter - prosecuting gambling site operators in other countries.
http://www.pokernewsdaily.com/u-s-could-face-world-trade-org...
"Two years later, the WTO ruled that the U.S. had violated international agreements on trade in services by prosecuting the operators of offshore internet gambling sites. The WTO rejected the U.S. stance that the restrictions were necessary to protect public morality and Antigua was subsequently awarded a settlement of $21 million from the U.S. as compensation."
Extradition is also by no means assured - Calvin Ayre is still quite comfortable up in Canada, despite the indictment and extradition requests.
UIGEA is also more of a problem for banking institutions. Online poker, for example, is not illegal in the US. The UIGEA is largely banking regulations, and these sites are explicitly blocking US users because they cannot interact with US banks.
We also have poker sites returning in several states, such as NJ, NV, etc.
But in general, these big international gambling sites don't deal with US players because financially it makes no sense, and many of them want to visit the US without getting arrested. It's not because they think they're going to get extradited on the next plane flight.
To bring it back around to the Brazil discussion, I doubt there's as much concern in the WhatsApp offices about not being able to visit Brazil.