As an American citizen, you are subject to its laws, even abroad.
Even if its legal for citizens of Z to do X in country Z, If you get caught doing it and come back to America, where X is illegal, you will be arrested for doing X.
AFAIK, This is really only gone after for major drugs and sex crimes. As long as you’re not stupid enough to announce it to the customs agents, or posted videos of you doing it all over the internet and draw massive attention to yourself, no one will typically care if you ate a pot brownie in Amsterdam