SakashiroFrom Galnet (emphasis mine):
Under Imperial law, citizens are not permitted to sell their Imperial Slaves for service to non-Imperial citizens, nor are they allowed to sell Imperial Slaves on worlds outside of Imperial space. Those found breaking the law run the risk of being forced into service themselves
Forced into voluntary slavery!
"Being forced into service themselves" means in this instance that those who abuse Imperial slaves may be punished by a court to slavery themselves. It's meant as a punishment and deterrent to make sure slaves do not get abused.