Why does the American Government (seem to) prevent the prosecution of Americans abroad?

You sound like somebody who has at least taken one political science cl. So you understand that the United States is the hegemonic power right now, regardless of what anyone says about China or India or whatever. We have the money, the military, and the ruthlessness and determination to get things done. Sometimes, this does result in shady doings. What you described seems shady at best and things like this make me shudder. However, since there is no real international law and we have the power to make our law THE law, then why not? Also, this is beneficial to Americans abroad because a lot of countries in the throes of political turmoil have corrupt police forces or drug cartels or whatever, you name it. Flashing your American pport gives you a lot of power, and consequently a little bit more safety, while abroad.