@morbiuswilters said:
1) It's illegal for a US citizen to use controlled substances, even outside the US.
That's close but not true. Drug distribution, use, etc is regulated at a federal level by the Controlled Substances Act of 1970. It is a territorial criminal statute just like most other US criminal statutes (meaning the offence has to take on US soil with some exceptions like military and ships).
Last year, the house of representatives passed the Drug Trafficking Safe Harbor Elimination Act of 2011 which would give US prosecutors the authority to prosecute certain violations of the Controlled Substances Act (mostly drug trafficking/distribution related) abroad regardless of the violator's citizenship status. This bill has not been voted on by the Senate yet, so it is not law.
And of course some controlled substances (e.g. medical drugs) are legal to use with a prescription.