Many other countries have much harsher laws against public intoxication, alcohol abuse, and use or possession of drugs. Legal sanctions pertaining to the use and possession of even the smallest amounts of illegal substances tend to be extremely severe.
U.S. authorities have no control or ability to assist a U.S. citizen found to be in violation of local law. In many countries, no distinction is made between personal use or possession and distribution or sale; in many developing countries, penalties can be life imprisonment or execution. Even associating with drug users can be very dangerous. U.S. laws and due process do not protect you abroad.