First of all, this is about Web sites, and that's not a valid analogy. Web sites don't spontaneously send you letters, let alone kidnap you. You only get content you actively asked for.
Second, where is the person writing the letter from? Mars? They're going to have a hard time finding a place where kidnapping and extortion are legal.
Third, the letter would in fact presumably be aimed at a specific person in a specific country... as would the kidnapping.
Websites absolutely spontaneously send letters, or advertising flyers.
The company could be from china or Russia with little interest in diplomatic pandering for such a small incident.
Providing a user a service in exchange for payment is also aimed at a specific person in a specific country.
heck, if no laws can be applied across borders, it could be a website selling the service of fake extortion letters.
And don't mix up "difficult to enforce" with "legal". Constantly changing domains and hiding who is behind the service are efforts to avoid being caught by very real and enforced laws.
Second, where is the person writing the letter from? Mars? They're going to have a hard time finding a place where kidnapping and extortion are legal.
Third, the letter would in fact presumably be aimed at a specific person in a specific country... as would the kidnapping.