For a gouvernement, Ajins are threats since they can't be killed. So they quite naturally label them as non humans so that they can treat them as they wish, without any treaty protecting the ajins. They lose any civil rights and almost nobody complains out of fear and maybe some jealousy too.
As for your remark to the fact they are humans that got possessed, well it might be we're the only ones to know that thanks to Kei's memories.
For the setting, well it's a point of view, I think there's enough matter to make it very entertaining in relation to moral, philosophy, civil rights, politics and so on. Maybe there's even too much and they might fail telling a good story because of that, in the limited time a show offers. But there's plenty of nice potential.
Maybe we should start a thread or add to it if it exists