It's not, although rules vary by state. Medical professionals can prescribe it even though it's a Schedule III drug. I get ads for these presumably legitimate therapy services online all the time on major social networks.

should Netflix fire all women on maternity leave, since they most likely were administered fentanyl during delivery ?

why isn't Netflix firing anyone who had even a minor surgery or even tooth extraction done, since it most likely involved opioids

Medical professionals can also use fentanyl among other drugs that aren't legal for recreational use.