Ancient slaves also worked hard to benefit their owners. In addition, Roman and Egyptian slaves were more servile and respectful and less rebellious than old south slaves in the United States. 

Also, although ancient slaves were less rebellious, they did meekly accept punishment via the whip, rod, or other means.  An ancient Egyptian inscription has a Egyptian taskmaster saying, "The rod is in my hand, be not idle."  Even the most generous ancient slave owner was willing to punish their slaves if necessary.