Not US, Australia, but wouldn't be surprised if same rules apply there – suppose you are prescribed 50mg Vyvanse. You start to feel like 50mg is too much (the side effects are too bad), and you want to try 40mg instead. Can the pharmacist dispense you 40mg instead of 50mg? No. Because it is a controlled substance (or "Schedule 8" as it is called in Australia), the pharmacist can only legally dispense the exact thing you were prescribed – they'll tell you to go back to the prescriber and get a new prescription. If it wasn't a controlled substance, the pharmacist would have more flexibility; in the past, I've gotten pharmacists to dispense prescription drugs without a prescription because I hadn't got around to getting a new script – something they are legally allowed to do (at least here in Australia) for non-controlled substances. They can't do that again and again – it is important the prescriber is kept in the loop – but as a short-term once-off, it does no harm. But going back to the prescriber involves getting another appointment, taking the time out of one's day to attend it, paying for it, etc. In the end one might just decide to put up with the side effects–which is a clearly less than optimal health outcome. Or, one can be reduced to trying to cut the dosage one's self – open the capsule, dissolve the contents in a measured quantity of liquid, then measure out the liquid – which will seem like too much effort to bother with.
I can understand how someone who is prescribed 50mg asking for 60mg or 70mg could be a sign of a developing addiction, but asking for less than one is prescribed is the complete opposite of how an addicted person behaves. So why isn't it allowed? Because the rules only have a rather diffuse relationship with preventing addiction (or other health harms). I think, to a great extent, the rules have come to exist for their own sake, not for the sake of anyone's health.