Most international pharmaceutical companies have some presence in the US, so the US quota has a world-wide effect.
Additionally, prescriptions are for very specific doses of specific variants of the meds. Because it's a controlled substance, pharmacies aren't allowed to use any substitutes (not even something common-sense like dispensing 2x30mg for a 60mg prescription). This makes shortages happen even before all of the quota runs out, because some commonly used doses run out sooner.