I was making a general statement, but will be the first to admit I don't see things in black and white. Your example seems perfectly legitimate.
Compare that to this situation:
I was working the weekend with just the one pharmacist. She called an acquaintance, a doctor, to request a prescription for something. She took the prescription over the phone, handed it to me, approved my typing of the script but added PRN refills since "that's what he would have wanted me to put." She then picked the exact NDC, etc, to use, and "verified" the filled prescription.
The prescription described above was benign, but it just seems like a flagrant disregard for professional behavior, and it's a potential occurrence when you tell people it's fine to do their own fills.
Obviously, not all scenarios will play out this way, but it doesn't take many before a company comes up with draconian measures to ensure it doesn't happen any more.