and see we could speculate as to why they chose to leave it or many other things out of the letter to the gentiles,but whether we agree with it or not in the letter it says "it seemed good both to the Holy Ghost and to us" so who are we do argue with the Holy ghost or the apostles,elders and brethren? I think that we are so accustomed to debating in every other thread that we assume we are to in every thread,,as for me I cannot add to or take away from what was written in the letter.
This was in a world where there were many Christians, most of them Torah Observant Jews. Diet, circumcision, the bathing, the whole shebang, and fully children of the king, they had taken Christ as their savior. They lived peacefully with those Jews who didn't accept Christ, went to the same synagogue. The gentile Christians who didn't want to do the physical things they did were in the minority. And the Observant Christians didn't want the non observant gentile Christians in their synagogue to learn of the true God. Read the entire story!!
Paul wanted them in the synagogue and wanted them there without having to do the diet or cutting of flesh.
The Jews took Paul to court (read the book of Acts!!!) and convicted him of teaching against Moses by teaching this, it was a death penalty. The Christian court led by James the Great, brother of Christ, ruled that Paul wasn't teaching against Moses (men today still say he does) but going to synagogue was so important that gentiles better do enough to get them accepted there, Moses was taught there.
You have posted what they were to do to get to synagogue, but you are leaving out all the other parts of the story, even the verse just following what they were to do to get there. It makes the message of those verses all distorted.