Why could God allow certain Law prohibitions to continue under the Gospel of Grace?
God allowed prohibitions against meat offered to idols, blood, things strangled, and fornication to continue because they were fundamental issues He still holds people accountable for, even under Grace. The Jerusalem agreement did not force circumcision or the whole Law, but these abstinences remained because they countered pagan and Satan-worship practices.
Les teaches that the Jerusalem church sent a written agreement to Antioch, deciding they would no longer force Gentile converts to keep the Law and practice circumcision. Yet they still asked them to abstain from meat offered to idols, blood, things strangled, and fornication. Les points out these instructions go back to Leviticus 17-18. Even though they were part of the Law, God could sanction them continuing with the Gospel of Grace because He still holds people accountable for these things. He also notes Satan worship often begins with drinking blood and immorality.