Atticus Finch is a good employer because he has integrity, honor, loyalty and he is a gentleman.  Calpurnia has worked for the Finch family for a long time.  Atticus trusts and respects her.  He trusts her to do what is right and good for his children and to run his household.  She has been the "woman of the house" since his wife died.  Atticus treats her fairly, pays her fairly, and makes Scout and Jem do as Cal tells them.  He gives her time off when she needs it and he treats her in an honorable manner. 

At one point in the story Atticus's sister tells him that he should let Calpurnia go; that she isn't needed any more.  Atticus tells his sister that Calpurnia is part of the family and that she can stay as long as she chooses to stay.