There are a number of forces preventing this:
1. US law requires attending radiologists to be trained in the US. Most US trained radiologists do not want to move out of the US. For example, the Indian telerad companies have had a hard time recruiting even Indian born US radiologists to move to India.
2. Even if a US trained and ACR boarded radiologist reads studies overseas, he can only submit a preliminary report. The law requires that the final report must be approved by a radiologist based in the US.
3. Referring physicians like to develop relationships with radiologists they trust. If a radiologist is convinced based on a CT study that a patient has epiploic appendagitis (non-surgical) and not appendicitis (surgical), the surgeon will not operate. Of course, the surgeon has to have a lot of faith in the radiologist b/c if he is wrong, badness (read "death") can happen. It's dfficult to develop faith in someone you never meet face-to-face.
Telerads has, for the most part, been great to US based radiologists by allowing them to sleep at home when on call. Of course, if the server goes down or the NightHawk company gets bogged-down, you get called in; but that's a rare event. I was once concerned about "outsorcing" of radiology, but not any more. Do your own research and I think you'll come to the same conclusion.