DEA numbers have nothing to do with it. DEA numbers are only necessary to write controlled substance prescriptions, and if the doctor does not need to write them in the course of their normal practice, they may elect not to register with the DEA. And now with the NPI numbers, insurance is no longer tied to DEA number (though there were ways around that too).
What is necessary is for the prescriber to be licensed somewhere in the United States. State medical boards have the authority to regulate who prescribes what, and license prescribers, not the DEA, which only regulates controlled substances. Two entirely different bodies. 🙂
Interesting about Puerto Rico. I need to read up on that...