Why are non-existent atrocities/acts of aggression more punishable than ACTUAL atrocities/acts of aggression?

Serbia, Vietnam, Iraq, Afghanistan, Panama, Nicaragua, Cuba, Libya etc are just some of the nations which have felt the force of US military action in recent years due to non-existent genocides or alleged acts of aggression. Yet incidences of actual genocide or mass murder (Rwanda, East Timor, Palestine etc) have resulted in precisely zero action; in fact Israel & Indonesia got most of the hardware they used to kill civilians from the USA or Britain. Is the lesson here that you can kill as many people as you wish, as long as you toe the US line....but if you don't toe the line then you are going to end up in the crosshairs, whether you've done something wrong or not?