The So-Called Slave Bible

Luke 17, chapter 17, verse 7: Who among you would say to your slave who is just coming from plowing or tending sheep in the field, come here at once and take your place at the table? This is sort of a message about why would you be thankful for things that you ought to have done anyway. And almost unthinkingly, the example that Jesus used is that of a slave. It seems strange to me that he would seemingly unthinkingly use slavery as a metaphor here as part of his spiritual guidance.