A previous answer states that, "God worked within the fallen system of man and put limits and guidelines concerning the treatment of slaves."
If that statement were correct, wouldn't everything else (rape, violence, stealing, etc.) that was common at that time be also acceptable to God? For example, if stealing was common at that time, then by the answer above that I quoted, God would put limits and guidelines concerning stealing, since it was within the fallen system of man at that time. It sounds like God then is a changing God, which is something we do not believe.
Why wouldn't God just make a statement saying that beating slaves or anyone is wrong regardless of how the culture or system of man is?
God worked through the fallen nature, e.g. rape, but the word "acceptable to God" in your question is invalid. God deals with all sin until He reforms us and take us to His image and likeness. God is most just and compassionate. There were many circumstances that evolved over time and God appointed laws to ensure civility amongst all people. When slavery became an issue, God declared righteous judgment in the treatment of slaves and all hired persons. When rape and other acts of violence surfaced, God ordered decrees of justice. To marry a rapist was up to the discretion of the father of the unwed girl, because remaining single following such an experience would be even more detrimental in some cases during those times and culture. The crying out of the victim was understood to mean the sexual act was not consensual. That being said, nowhere in the Holy Scripture does God sanction or condone rape at all. The penalty of rape was death (Deuteronomy 22:22-29).
All laws became constitutional for the Israelites. God provided the guidelines for the people to put a judicial system in place and to provide fair consequences and reprimand for breaking the rules and laws. These same laws—which are the Ten Commandments, unto this day guard and protect society, even here in the United States. There are at least eight etchings of the archprophet Moses in the ceiling of the Supreme Court Great hall. On the support structure of the courtroom's bronze doors and in the library carpentry, are engraved tablets representing the Ten Commandments and directly above the head of the chief justice is a plaque revealing Roman numerals I through X.