States have a legitimate right to taxation. Taxes can be used to pay volunteers. While there are still people owning wealth above the poverty level, there is no excuse to force people into military slavery. And even if literally everybody has been taxed into poverty, there are more ethical alternatives than slavery, e.g. tying the right to vote to military service like in Starship Troopers. Freedom from slavery is extremely high on the scale of ethical importance, even more important than democracy.