The problem is that you can't separate the economic from the social---even a mutually beneficial transaction can have negative social externalities like undermi

The problem is that you can't separate the economic from the social---even a mutually beneficial transaction can have negative social externalities like undermining the political (and then the economic) power of a whole class of workers or potentially even undermining the individual capacity for independence of the individual involved on a psychological level. The philosopher Debra Satz's book "Why Some Things Should Not be for Sale" has a great section on bonded labor that captures some of these consequences. (I think the military isn't a good analogy because the public at large affords military personnel a special kind of respect that counteracts these social externalities.)