Earlier quoted context omitted.
I'm guessing because a Google search of "Adderall side effects" immediately returned this: "Rare but severe side effects of Adderall can include emotional instability, psychiatric disorders and cardiovascular events. Adderall misuse can lead to seizures and death."
There are rare but terrible side effects for virtually all medications. What is your point?
There are plenty of medicines that warn against operating motor vehicles or heavy machinery, off the top of my head some blood pressure meds like Losartan and HCTZ because they can make you dizzy/prone to blacking out. I imagine there's similar logic here for pilots.