Acts 15:21

"For": The reason that Gentile Christians need such instruction is because, in contrast the to Jews who had heard the Scriptures read from ancient generations every week, the Gentiles have not had access to such instruction concerning fornication and the above. Jewish Christians do not need such instruction because they had been properly taught, but the Gentile Christians, who are without written revelation (in the past), need to be given these prohibitions. Obviously, James is not saying that everyone is still under the Law of Moses, because verses 7-19 made the opposite point.