I haven’t really heard the words of “attack and enslave law-abiding civilian population” part myself!
Maybe not of them are so law-abiding as it was made to believe? We know that Empire lately was shaken by a serie of revolts, ignited by Blood Raiders and other activists. I cannot say anything about the population of the planet and percentage of criminals down there, but I do have Faith in Imperial command to separate the wheat from the chaff. After all, they aren’t planning to go full gallente on the population and just genocide them. They’re going to capture them, and if they capture them they can process them by legal procedures. After all, what I got from Imperial practises, slavery is a form of legal punishment for something, and that means they should prove each of citizens have committed that something in order to be sentenced to slavery, IMHO.
The fact of capturing someone does not constitute a war crime by itself, because if you capture them - you can release them. Crime would be holding them as hostages or killing them. I am pretty sure if nobody will resist the authority - nobody will get hurt.