@xyzzy dude, im not saying its okay to do it, but people here need to understand that there is a difference between legal and ethical.
The thought process you arrived at is based on common sense from this world, in the XX century. Its really easy to see the difference in ethical values if you just go like, 100 years back, or even 200 where slavery was still legal AND ethical, and by todays standards thats an abomination, which serves to prove my point that using ethics to determine if something is good or bad is a complicated process.
Sure, we all agree that killing is bad, or stealing or raping, but people still cant agree if gay marriage is okay or not, or if abortion is okay or not, so ethics are not an iron rule, add to the fact you are dealing with a different world, with different values and a different common sense, and your ethic no longer holds water.
What im trying to say is, at some point in time, the other worlds races are going to demand to have the same rights as humans, and based on the fact they are sentient, sapient beings, it wouldn't be hard to think those rights are either gonna be recognized or the definition of "human" is going to broaden in order to include them.
But until that happens, there is no "war crime". Doesn't mean what they are doing is acceptable to modern society´s eyes, im sure they would get a lot of heat from the public opinion and even other countries for it, but it still isnt illegal, and therefore, its not a war crime.