The argument about this being right is the basic argument about Lawful vs. Morality. Because it's a law,does it make it right? Or does morality dictate what's right? If so, then whose Morality? Should Laws be based on Morality, or Order? Everyones views on this will differ based on how lawful or how moral one is.
Also, let's not forget *WHY* tuon (and the seanchan) think the way they do. In their past, Channelers cause many deaths and wars, so in an effort to control them, they were bound to slavery. As a society, they have accepted this as a necessary evil to prevent harm to the innocent. In fact, their society has accepted it to the point where it's almost considered an honor of sorts to be forced into a certain lifestyle (slave or master) and people who she the correct signs accept it with humility. Naturally, OTHER societies are appalled by it, it's kind of like how some eastern countries would find us monsters for eating beef.
So lets look at it from the perspective in ancient times. There's a group of people. A vast majority (let's say 80-90%) of them are blood thirsty and murderous, and harm your innocent citizens. These people are also superior to you in a way that you CANNOT fight them back, no matter what you do, and they know it. As such, they have no reason at all to bargain with you. However, you have a certain tool that will allow you to bind them to slavery, and keep them in check. Doing so, you keep this group of people from harming the innocent, but requires you to bind all of them in order to be sure of the safety. Even one of these people on a rampage could kill hundreds or thousands. Would you choose to bind them to slavery, or allow the innocent to suffer by their hands?