Currently, there is a major issue involving the overuse of opioid painkillers in pain management. So much so that the FDA has released guidelines for doctors to explore the use of chiropractic methods and acupuncture as alternative treatments:
https://www.fda.gov/downloads/Drugs/NewsEvents/UCM557071.pdf

The FDA has also approved an electroacupuncture system in 2014 to help eliminated opioid withdrawal symptoms:
https://www.fda.gov/NewsEvents/Newsroom/PressAnnouncements/ucm585271.htm

From these two sources it seems that the FDA sees potential in the methods as a way to help combat opioid addiction, which currently is a major epidemic in the US. The FDA may feel that the potential benefits outweigh the current lack of long term clinical data. I would agree with this sentiment as the low voltages used pose a very small risk to the patient.