It is also worth noting that meth is not typically prescribed for ADHD, it's typically other amphetamines like Adderall that are, or methylphenidate (Ritalin). Prescription meth (Desoxyn) is typically only prescribed for some things like eating disorders, usually not ADHD. And it's very distinct from the other amphetamines, they're not really directly comparable.

That said, plenty of people with ADHD do actually self-medicate with (black market!) meth and are perfectly functional. To be quite frank, the stigma surrounding meth use is simply harmful in all but the most egregious cases. And even in those cases, usually it's not the meth itself that is the problem, it's simply the favorite substance of the addict (who would've otherwise just gotten addicted to something else instead).

Another often-overlooked aspect is that addicts usually only get addicted to substances because their life already sucks, and taking away the substances without helping their life at all is often just cruel, and especially blaming them for trying to cope with their circumstances. So many members of society use such simple (and flawed) logic, seeing someone using drugs and assuming the drugs are the problem, and that taking it away will make it better. Usually both are false (though some notable exceptions are drugs that form strong dependence like opioids and benzodiazepines, and other drugs that pose a strong risk to life, like research/designer psychedelics -- ones that are not safe like LSD/DMT or psilocybin -- or deliriants)