My understanding is because it is sold as a food supplement in the U.S while the UK, for example regulates it as a medicine. So its available for prescription

My understanding is because it is sold as a food supplement in the U.S while the UK, for example regulates it as a medicine. So its available for prescription for short-term treatment of sleep disordersm, with provisos such as: "Melatonin’s metabolism is mainly mediated via CYP1A enzymes. Therefore interactions may occur between melatonin and other drugs affecting or metabolised by CYP1A enzymes e.g. fluvoxamine, cimetidine, ciprofloxacin and other quinolones, carbamazepine, omeprazole, oestrogens (oral contraceptives and HRT), rifampicin, theophylline and increase in blood pressure with nifedipine and reduced prothrombin with warfarin have been reported." etc.